diff --git a/spec/README.md b/spec/README.md
index 76cc998cf8..c603282ca4 100644
--- a/spec/README.md
+++ b/spec/README.md
@@ -17,6 +17,7 @@
    1. [Resolution Errors](errors.md#resolution-errors)
    1. [Message Function Errors](errors.md#message-function-errors)
 1. [Default Function Registry](registry.md)
+1. [`u:` Namespace](u-namespace.md)
 1. [Formatting](formatting.md)
 1. [Interchange data model](data-model/README.md)
 
diff --git a/spec/formatting.md b/spec/formatting.md
index 27495fb01e..f1a12cae08 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -260,9 +260,22 @@ the following steps are taken:
 
 3. Perform _option resolution_.
 
-4. Call the _function handler_ with the following arguments:
+4. Determine the _function context_ for calling the _function handler_.
 
-   - The current _locale_.
+   The **_<dfn>function context</dfn>_** contains the context necessary for
+   the _function handler_ to resolve the _expression_. This includes:
+
+   - The current _locale_,
+     potentially including a fallback chain of locales.
+   - The base directionality of the _message_ and its _text_ tokens.
+
+   If the resolved mapping of _options_ includes any _`u:` options_
+   supported by the implementation, process them as specified.
+   Such `u:` options MAY be removed from the resolved mapping of _options_.
+
+5. Call the function implementation with the following arguments:
+
+   - The _function context_.
    - The resolved mapping of _options_.
    - If the _expression_ includes an _operand_, its _resolved value_.
 
@@ -272,7 +285,7 @@ the following steps are taken:
    as long as reasonable precautions are taken to keep the function interface
    simple and minimal, and avoid introducing potential security vulnerabilities.
 
-5. If the call succeeds,
+6. If the call succeeds,
    resolve the value of the _expression_ as the result of that function call.
 
    If the call fails or does not return a valid value,
@@ -345,6 +358,10 @@ The _resolved value_ of _markup_ includes the following fields:
 - The _identifier_ of the _markup_
 - The resolved _options_ values after _option resolution_.
 
+If the resolved mapping of _options_ includes any _`u:` options_
+supported by the implementation, process them as specified.
+Such `u:` options MAY be removed from the resolved mapping of _options_.
+
 The resolution of _markup_ MUST always succeed.
 
 ### Fallback Resolution
diff --git a/spec/u-namespace.md b/spec/u-namespace.md
new file mode 100644
index 0000000000..dabbcc70f2
--- /dev/null
+++ b/spec/u-namespace.md
@@ -0,0 +1,87 @@
+# MessageFormat 2.0 Unicode Namespace
+
+The `u:` _namespace_ is reserved for the definition of _options_
+which affect the _function context_ of the specific _expressions_
+in which they appear,
+or for the definition of _options_ that are universally applicable
+rather than function-specific.
+It might also be used to define _functions_ in a future release.
+
+The CLDR Technical Committee of the Unicode Consortium
+manages the specification for this namespace, hence the name `u:`.
+
+## Options
+
+This section describes common **_<dfn>`u:` options</dfn>_** which each implementation SHOULD support
+for all _functions_ and _markup_.
+
+### `u:id`
+
+A string value that is included as an `id` or other suitable value
+in the formatted parts for the _placeholder_,
+or any other structured formatted results.
+
+Ignored when formatting a message to a string.
+
+The value of the `u:id` _option_ MUST be a _literal_ or a
+_variable_ whose _resolved value_ is either a string
+or can be resolved to a string without error.
+For other values, a _Bad Option_ error is emitted
+and the `u:id` option is ignored.
+
+### `u:locale`
+
+Replaces the _locale_ defined in the _function context_ for this _expression_.
+
+A comma-delimited list consisting of
+well-formed [BCP 47](https://www.rfc-editor.org/rfc/bcp/bcp47.txt)
+language tags,
+or an implementation-defined list of such tags.
+
+If this option is set on _markup_, a _Bad Option_ error is emitted
+and the value of the `u:locale` option is ignored.
+
+During processing, the `u:locale` option
+MUST be removed from the resolved mapping of _options_
+before calling the _function handler_.
+
+Values matching the following ABNF are always accepted:
+```abnf
+u-locale-option = unicode_bcp47_locale_id *(o "," o unicode_bcp47_locale_id)
+```
+using `unicode_bcp47_locale_id` as defined for
+[Unicode Locale Identifier](https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#unicode_bcp47_locale_id).
+
+Implementations MAY support additional language tags,
+such as private-use or grandfathered tags,
+or tags using `_` instead of `-` as a separator.
+When the value of `u:locale` is set by a _variable_,
+implementations MAY support non-string values otherwise representing locales.
+
+Implementations MAY emit a _Bad Option_ error
+and MAY ignore the value of the `u:locale` _option_ as a whole
+or any of the entries in the list of language tags.
+This might be because the locale specified is not supported
+or because the language tag is not well-formed,
+not valid, or some other reason.
+
+### `u:dir`
+
+Replaces the base directionality defined in
+the _function context_ for this _expression_.
+
+If this option is set on _markup_, a _Bad Option_ error is emitted
+and the value of the `u:dir` option is ignored.
+
+During processing, the `u:dir` option
+MUST be removed from the resolved mapping of _options_
+before calling the _function handler_.
+
+The value of the `u:dir` _option_ MUST be one of the following _literal_ values
+or a _variable_ whose _resolved value_ is one of these _literals_:
+- `ltr`: left-to-right directionality
+- `rtl`: right-to-left directionality
+- `auto`: directionality determined from _expression_ contents
+
+For other values, a _Bad Option_ error is emitted
+and the value of the `u:dir` option is ignored.
diff --git a/test/README.md b/test/README.md
index 6fa50dfc50..d5cbee831c 100644
--- a/test/README.md
+++ b/test/README.md
@@ -10,6 +10,8 @@ These test files are intended to be useful for testing multiple different messag
 - `data-model-errors.json` - Strings that should produce a Data Model Error when processed.
   Error names are defined in ["MessageFormat 2.0 Errors"](../spec/errors.md) in the spec.
 
+- `u-options.json` — Test cases for the `u:` options, using built-in functions.
+
 - `functions/` — Test cases that correspond to built-in functions.
   The behaviour of the built-in formatters is implementation-specific so the `exp` field is often
   omitted and assertions are made on error cases.
@@ -21,6 +23,7 @@ Some examples of test harnesses using these tests, from the source repository:
 - [Formatting tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/messageformat.test.ts)
 
 A [JSON schema](./schemas/) is included for the test files in this repository.
+
 ## Error Codes
 
 The following table relates the error names used in the [JSON schema](./schemas/)
diff --git a/test/schemas/v0/tests.schema.json b/test/schemas/v0/tests.schema.json
index a0dd0a56e1..a37dcfa8db 100644
--- a/test/schemas/v0/tests.schema.json
+++ b/test/schemas/v0/tests.schema.json
@@ -269,6 +269,9 @@
               "name": {
                 "type": "string"
               },
+              "id": {
+                "type": "string"
+              },
               "options": {
                 "type": "object"
               }
diff --git a/test/tests/u-options.json b/test/tests/u-options.json
new file mode 100644
index 0000000000..3e13b30a24
--- /dev/null
+++ b/test/tests/u-options.json
@@ -0,0 +1,126 @@
+{
+  "$schema": "https://raw.githubusercontent.com/unicode-org/message-format-wg/main/test/schemas/v0/tests.schema.json",
+  "scenario": "u: Options",
+  "description": "Common options affecting the function context",
+  "defaultTestProperties": {
+    "locale": "en-US"
+  },
+  "tests": [
+    {
+      "src": "{#tag u:id=x}content{/ns:tag u:id=x}",
+      "exp": "content",
+      "expParts": [
+        {
+          "type": "markup",
+          "kind": "open",
+          "id": "x",
+          "name": "tag"
+        },
+        {
+          "type": "literal",
+          "value": "content"
+        },
+        {
+          "type": "markup",
+          "kind": "close",
+          "id": "x",
+          "name": "tag"
+        }
+      ]
+    },
+    {
+      "src": "{#tag u:dir=rtl u:locale=ar}content{/ns:tag}",
+      "exp": "content",
+      "expErrors": [{ "type": "bad-option" }, { "type": "bad-option" }],
+      "expParts": [
+        {
+          "type": "markup",
+          "kind": "open",
+          "name": "tag"
+        },
+        {
+          "type": "literal",
+          "value": "content"
+        },
+        {
+          "type": "markup",
+          "kind": "close",
+          "name": "tag"
+        }
+      ]
+    },
+    {
+      "src": "hello {4.2 :number u:locale=fr}",
+      "exp": "hello 4,2"
+    },
+    {
+      "src": "hello {world :string u:dir=ltr u:id=foo}",
+      "exp": "hello world",
+      "expParts": [
+        {
+          "type": "literal",
+          "value": "hello "
+        },
+        {
+          "type": "string",
+          "source": "|world|",
+          "dir": "ltr",
+          "id": "foo",
+          "value": "world"
+        }
+      ]
+    },
+    {
+      "src": "hello {world :string u:dir=rtl}",
+      "exp": "hello \u2067world\u2069",
+      "expParts": [
+        {
+          "type": "literal",
+          "value": "hello "
+        },
+        {
+          "type": "string",
+          "source": "|world|",
+          "dir": "rtl",
+          "value": "world"
+        }
+      ]
+    },
+    {
+      "src": "hello {world :string u:dir=auto}",
+      "exp": "hello \u2068world\u2069",
+      "expParts": [
+        {
+          "type": "literal",
+          "value": "hello "
+        },
+        {
+          "type": "string",
+          "source": "|world|",
+          "dir": "auto",
+          "value": "world"
+        }
+      ]
+    },
+    {
+      "locale": "ar",
+      "src": "أهلاً {بالعالم :string u:dir=rtl}",
+      "exp": "أهلاً \u2067بالعالم\u2069"
+    },
+    {
+      "locale": "ar",
+      "src": "أهلاً {بالعالم :string u:dir=auto}",
+      "exp": "أهلاً \u2068بالعالم\u2069"
+    },
+    {
+      "locale": "ar",
+      "src": "أهلاً {world :string u:dir=ltr}",
+      "exp": "أهلاً \u2066world\u2069"
+    },
+    {
+      "locale": "ar",
+      "src": "أهلاً {بالعالم :string}",
+      "exp": "أهلاً \u2067بالعالم\u2069"
+    }
+  ]
+}