diff --git a/harness/temporalHelpers.js b/harness/temporalHelpers.js
index 113db3da67..91f05cbd8a 100644
--- a/harness/temporalHelpers.js
+++ b/harness/temporalHelpers.js
@@ -2020,6 +2020,9 @@ var TemporalHelpers = {
         "11-18junk",
         "11-18[u-ca=gregory]",
         "11-18[u-ca=hebrew]",
+        "11-18[U-CA=iso8601]",
+        "11-18[u-CA=iso8601]",
+        "11-18[FOO=bar]",
       ];
     },
 
@@ -2107,6 +2110,11 @@ var TemporalHelpers = {
     plainYearMonthStringsInvalid() {
       return [
         "2020-13",
+        "1976-11[u-ca=gregory]",
+        "1976-11[u-ca=hebrew]",
+        "1976-11[U-CA=iso8601]",
+        "1976-11[u-CA=iso8601]",
+        "1976-11[FOO=bar]",
       ];
     },
 
diff --git a/test/built-ins/Temporal/Calendar/prototype/dateAdd/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/dateAdd/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..718b83c6bd
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/dateAdd/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.dateadd
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.dateAdd(arg, new Temporal.Duration()),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/dateUntil/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/dateUntil/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..28260458d4
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/dateUntil/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,27 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.dateuntil
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.dateUntil(arg, new Temporal.PlainDate(1977, 11, 19)),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => instance.dateUntil(new Temporal.PlainDate(1977, 11, 19), arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/day/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/day/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..c883f62f1e
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/day/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.day
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.day(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/dayOfWeek/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/dayOfWeek/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..67defe4d91
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/dayOfWeek/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.dayofweek
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.dayOfWeek(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/dayOfYear/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/dayOfYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..11583c2699
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/dayOfYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.dayofyear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.dayOfYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/daysInMonth/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/daysInMonth/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..be5615d56f
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/daysInMonth/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.daysinmonth
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.daysInMonth(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/daysInWeek/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/daysInWeek/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ea1697559e
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/daysInWeek/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.daysinweek
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.daysInWeek(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/daysInYear/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/daysInYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8720589b58
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/daysInYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.daysinyear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.daysInYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/inLeapYear/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/inLeapYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8c3d445d7f
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/inLeapYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.inleapyear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.inLeapYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/month/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/month/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..872a8dd73d
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/month/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.month
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.month(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthCode/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/monthCode/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..829eda0614
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthCode/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.monthcode
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.monthCode(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..d04ea566ed
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.monthsinyear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.monthsInYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/weekOfYear/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/weekOfYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..0587d163db
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/weekOfYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.weekofyear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.weekOfYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/year/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/year/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..b6d34e38fe
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/year/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.year
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.year(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Calendar/prototype/yearOfWeek/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Calendar/prototype/yearOfWeek/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..b379ca8373
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/yearOfWeek/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.yearofweek
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.yearOfWeek(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Instant/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Instant/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..47b63154ee
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,29 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const epoch = new Temporal.Instant(0n);
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.Instant.compare(arg, epoch),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.Instant.compare(epoch, arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/Instant/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Instant/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..f965e0da01
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.Instant.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Instant/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Instant/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..c6b4928850
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Instant(0n);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Instant/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Instant/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..44a78f0567
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Instant(0n);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/Instant/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/Instant/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..217e034564
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Instant(0n);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8004b62da9
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,27 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDate.compare(arg, new Temporal.PlainDate(1976, 11, 18)),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDate.compare(new Temporal.PlainDate(1976, 11, 18), arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..d96700ba6a
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDate.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..a63e7b419c
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDate(2000, 5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ba42ce2430
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDate(2000, 5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8fb5465b94
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.toplaindatetime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDate(2000, 5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.toPlainDateTime(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8fd9ea7756
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.tozoneddatetime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDate(2000, 5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.toZonedDateTime({ plainTime: arg, timeZone: "UTC" }),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDate/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDate/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..148b04e1f7
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDate(2000, 5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..3f80f31711
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,27 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDateTime.compare(arg, new Temporal.PlainDateTime(1976, 11, 18)),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDateTime.compare(new Temporal.PlainDateTime(1976, 11, 18), arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..b735be7854
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainDateTime.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..4f4bd62ab6
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..dc2929175c
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..9fdf98b7d5
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..231a4bdefb
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.withplaindate
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.withPlainDate(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..bde8161a9f
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.withplaintime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.withPlainTime(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainMonthDay/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainMonthDay/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..21cd225fba
--- /dev/null
+++ b/test/built-ins/Temporal/PlainMonthDay/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainMonthDay.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainMonthDay/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainMonthDay/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..2d70e98e42
--- /dev/null
+++ b/test/built-ins/Temporal/PlainMonthDay/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainMonthDay(5, 2);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..6d6431aa1a
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,33 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainTime.compare(arg, new Temporal.PlainTime(12, 34, 56, 987, 654, 321)),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainTime.compare(new Temporal.PlainTime(12, 34, 56, 987, 654, 321), arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..51a4c42e60
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainTime.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..c109c395ad
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainTime(12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..f267f393c2
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainTime(12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..a43626e456
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/prototype/toPlainDateTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.toplaindatetime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainTime(12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.toPlainDateTime(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ca20c7806e
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/prototype/toZonedDateTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.tozoneddatetime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainTime(12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.toZonedDateTime({ plainDate: arg, timeZone: "UTC" }),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainTime/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..eeaaebf082
--- /dev/null
+++ b/test/built-ins/Temporal/PlainTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,28 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainTime(12, 34, 56, 987, 654, 321);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainYearMonth/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainYearMonth/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..d845def48d
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,27 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainYearMonth.compare(arg, new Temporal.PlainYearMonth(2019, 6)),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainYearMonth.compare(new Temporal.PlainYearMonth(2019, 6), arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainYearMonth/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainYearMonth/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..962cb8a391
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.PlainYearMonth.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainYearMonth/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainYearMonth/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8444ffd1dd
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainYearMonth(2000, 5);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainYearMonth/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainYearMonth/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ebdec3dd0a
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainYearMonth(2000, 5);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/PlainYearMonth/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/PlainYearMonth/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ff05b679cb
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.PlainYearMonth(2000, 5);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getInstantFor/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getInstantFor/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..0ab9cb35bc
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getInstantFor/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getinstantfor
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getInstantFor(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getNextTransition/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getNextTransition/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..c70b2259b5
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getNextTransition/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getnexttransition
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getNextTransition(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getOffsetNanosecondsFor/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getOffsetNanosecondsFor/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..72093a11d8
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getOffsetNanosecondsFor/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getoffsetnanosecondsfor
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getOffsetNanosecondsFor(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getOffsetStringFor/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getOffsetStringFor/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..85567f5e5d
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getOffsetStringFor/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getoffsetstringfor
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getOffsetStringFor(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getPlainDateTimeFor/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getPlainDateTimeFor/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..2708de707a
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getPlainDateTimeFor/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getplaindatetimefor
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getPlainDateTimeFor(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getPossibleInstantsFor/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getPossibleInstantsFor/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ebbf696eac
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getPossibleInstantsFor/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getpossibleinstantsfor
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getPossibleInstantsFor(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/TimeZone/prototype/getPreviousTransition/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/TimeZone/prototype/getPreviousTransition/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..24c567b1df
--- /dev/null
+++ b/test/built-ins/Temporal/TimeZone/prototype/getPreviousTransition/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone.prototype.getprevioustransition
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00Z[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00Z[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00Z[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.TimeZone("UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.getPreviousTransition(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/compare/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/compare/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..932470864a
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/compare/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,29 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.compare
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const datetime = new Temporal.ZonedDateTime(0n, "UTC");
+
+const invalidStrings = [
+  ["1970-01-01T00:00[UTC][U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[UTC][u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[UTC][FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.ZonedDateTime.compare(arg, datetime),
+    `annotation keys must be lowercase: ${arg} - ${descr} (first argument)`
+  );
+  assert.throws(
+    RangeError,
+    () => Temporal.ZonedDateTime.compare(datetime, arg),
+    `annotation keys must be lowercase: ${arg} - ${descr} (second argument)`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/from/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/from/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..b73df83840
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/from/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,29 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.from
+description: Annotation keys are lowercase-only 
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[UTC][U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[UTC][u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[UTC][FOO=bar]", "invalid capitalized unrecognized key"],
+];
+
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => Temporal.ZonedDateTime.from(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+  for (const offset of ["use", "prefer", "ignore", "reject"]) {
+    assert.throws(
+      RangeError,
+      () => Temporal.ZonedDateTime.from(arg, { offset }),
+      `annotation keys must be lowercase: ${arg} - ${descr} (offset ${offset})`
+    );
+  }
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..5384695e4f
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/equals/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.equals
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[UTC][U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[UTC][u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[UTC][FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.ZonedDateTime(0n, "UTC");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.equals(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..c1705471d4
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/since/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,23 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.since
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[UTC][U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[UTC][u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[UTC][FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const timeZone = new Temporal.TimeZone("UTC");
+const instance = new Temporal.ZonedDateTime(0n, timeZone);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.since(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..4375637118
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/until/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,23 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.until
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01T00:00[UTC][U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01T00:00[UTC][u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01T00:00[UTC][FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const timeZone = new Temporal.TimeZone("UTC");
+const instance = new Temporal.ZonedDateTime(0n, timeZone);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.until(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..4afe3bf794
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainDate/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,23 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.withplaindate
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const timeZone = new Temporal.TimeZone("UTC");
+const instance = new Temporal.ZonedDateTime(0n, timeZone);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.withPlainDate(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js b/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..32b59887c0
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/withPlainTime/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,29 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.withplaintime
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["00:00[U-CA=iso8601]", "invalid capitalized key, time-only"],
+  ["T00:00[U-CA=iso8601]", "invalid capitalized key, time designator"],
+  ["1970-01-01T00:00[U-CA=iso8601]", "invalid capitalized key"],
+  ["00:00[u-CA=iso8601]", "invalid partially-capitalized key, time-only"],
+  ["T00:00[u-CA=iso8601]", "invalid partially-capitalized key, time designator"],
+  ["1970-01-01T00:00[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["00:00[FOO=bar]", "invalid capitalized unrecognized key, time-only"],
+  ["T00:00[FOO=bar]", "invalid capitalized unrecognized key, time designator"],
+  ["1970-01-01T00:00[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const timeZone = new Temporal.TimeZone("UTC");
+const instance = new Temporal.ZonedDateTime(0n, timeZone);
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.withPlainTime(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/intl402/Temporal/Calendar/prototype/era/argument-string-calendar-annotation-invalid-key.js b/test/intl402/Temporal/Calendar/prototype/era/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..8d570e5b22
--- /dev/null
+++ b/test/intl402/Temporal/Calendar/prototype/era/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.era
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.era(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});
diff --git a/test/intl402/Temporal/Calendar/prototype/eraYear/argument-string-calendar-annotation-invalid-key.js b/test/intl402/Temporal/Calendar/prototype/eraYear/argument-string-calendar-annotation-invalid-key.js
new file mode 100644
index 0000000000..ac1c686e8b
--- /dev/null
+++ b/test/intl402/Temporal/Calendar/prototype/eraYear/argument-string-calendar-annotation-invalid-key.js
@@ -0,0 +1,22 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.erayear
+description: Annotation keys are lowercase-only
+features: [Temporal]
+---*/
+
+const invalidStrings = [
+  ["1970-01-01[U-CA=iso8601]", "invalid capitalized key"],
+  ["1970-01-01[u-CA=iso8601]", "invalid partially-capitalized key"],
+  ["1970-01-01[FOO=bar]", "invalid capitalized unrecognized key"],
+];
+const instance = new Temporal.Calendar("iso8601");
+invalidStrings.forEach(([arg, descr]) => {
+  assert.throws(
+    RangeError,
+    () => instance.eraYear(arg),
+    `annotation keys must be lowercase: ${arg} - ${descr}`
+  );
+});