diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 552ab9403..0c2906fe1 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Anthropic Messages tool schema normalization demoting root `anyOf`/`allOf` and all `oneOf` constraints into descriptions instead of forwarding provider-rejected keywords in MCP tool `input_schema`.
+
 ## [16.0.5] - 2026-06-17
 
 ### Added
diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index a75bbb259..0c25c3065 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -3265,8 +3265,9 @@ export function convertAnthropicMessages(
 /**
  * JSON Schema whitelist for Anthropic tool `input_schema` nodes.
  *
- * Mirrors the Anthropic Python SDK's `lib/_parse/_transform.py::transform_schema`:
- * we keep only structural/metadata keywords Anthropic's validator honors, and demote
+ * Tracks the Anthropic Python SDK's `lib/_parse/_transform.py::transform_schema`,
+ * with live Messages API guardrails for keywords the SDK preserves but the API rejects.
+ * We keep only structural/metadata keywords Anthropic's validator honors, and demote
  * anything else into the node's `description` as `\n\n{key: value, ...}` so the model
  * still sees the constraint as a natural-language hint.
  *
@@ -3281,7 +3282,6 @@ const ANTHROPIC_TOOL_SCHEMA_UNIVERSAL_KEEP = new Set([
 	"definitions",
 	"type",
 	"anyOf",
-	"oneOf",
 	"allOf",
 	"enum",
 	"const",
@@ -3377,13 +3377,16 @@ function anthropicPerTypeKeep(scalarType: string | undefined): Set<string> | und
  * Applies the full whitelist semantics from the Anthropic Python SDK's
  * `lib/_parse/_transform.py::transform_schema`:
  *
- * 1. Universal keys (`$ref`, `$defs`, `type`, `anyOf`/`oneOf`/`allOf`, `enum`, `const`,
- *    `description`, `title`, `default`, `nullable`) are preserved on every node.
+ * 1. Universal keys (`$ref`, `$defs`, `type`, `anyOf`, `allOf`, `enum`, `const`,
+ *    `description`, `title`, `default`, `nullable`) are preserved on every node, with
+ *    one position-dependent exception: the combinator keys. Root `anyOf`/`allOf` are
+ *    spilled (recent Anthropic Messages validators reject combinators at the tool
+ *    `input_schema` root) but kept when nested; `oneOf` is spilled at every position
+ *    (it is not in the documented supported subset).
  * 2. Per-type keys are kept additively (object → `properties`/`required`/`additionalProperties`,
  *    array → `items`/`prefixItems` plus `minItems` only when 0 or 1, string → `format`
  *    only when in the supported value set).
  * 3. Everything else is demoted into the node's `description` as `\n\n{key: value, ...}`
- *    so the model still sees the constraint as a natural-language hint.
  *
  * Object nodes default to `additionalProperties: false`, but explicit open-map
  * declarations (`additionalProperties: true` or a schema literal — Zod's
@@ -3394,6 +3397,7 @@ function anthropicPerTypeKeep(scalarType: string | undefined): Set<string> | und
 function normalizeAnthropicToolSchemaNode(
 	schema: unknown,
 	cache: WeakMap<Record<string, unknown>, Record<string, unknown>>,
+	isRoot = false,
 ): unknown {
 	if (Array.isArray(schema)) return schema.map(entry => normalizeAnthropicToolSchemaNode(entry, cache));
 	if (!isRecord(schema)) return schema;
@@ -3411,7 +3415,8 @@ function normalizeAnthropicToolSchemaNode(
 	for (const key in schema) {
 		if (!Object.hasOwn(schema, key)) continue;
 		const value = schema[key];
-		if (ANTHROPIC_TOOL_SCHEMA_UNIVERSAL_KEEP.has(key) || perTypeKeep?.has(key)) {
+		const isRootCombinator = isRoot && COMBINATOR_KEYS.includes(key as (typeof COMBINATOR_KEYS)[number]);
+		if (!isRootCombinator && (ANTHROPIC_TOOL_SCHEMA_UNIVERSAL_KEEP.has(key) || perTypeKeep?.has(key))) {
 			result[key] = value;
 		} else {
 			spill.push([key, value]);
@@ -3486,7 +3491,7 @@ function normalizeAnthropicToolSchemaNode(
 }
 
 export function normalizeAnthropicToolSchema(schema: unknown): unknown {
-	return normalizeAnthropicToolSchemaNode(schema, new WeakMap());
+	return normalizeAnthropicToolSchemaNode(schema, new WeakMap(), true);
 }
 
 type AnthropicToolSchemaPlan = {
diff --git a/packages/ai/test/anthropic-tool-schema.test.ts b/packages/ai/test/anthropic-tool-schema.test.ts
index e52e3225f..3837c6915 100644
--- a/packages/ai/test/anthropic-tool-schema.test.ts
+++ b/packages/ai/test/anthropic-tool-schema.test.ts
@@ -196,12 +196,14 @@ describe("normalizeAnthropicToolSchema — SDK whitelist", () => {
  * `anthropic-sdk-python/tests/lib/_parse/test_transform.py`. We adapt assertions
  * to the function name `normalizeAnthropicToolSchema` and keep the same shapes.
  *
- * Two deliberate divergences from the SDK (NOT bugs):
+ * Three deliberate divergences from the SDK (NOT bugs):
  *  - `default` is preserved on every node (SDK demotes it into description).
  *    Anthropic's API accepts `default`; preserving keeps Zod/OpenAPI fidelity.
  *  - `$ref` does NOT short-circuit sibling keys (SDK drops everything else).
  *    We keep `$defs`/`description` next to a `$ref` because callers feed us
  *    deref-friendly schemas where siblings carry real semantics.
+ *  - top-level `anyOf` / `allOf` and all `oneOf` occurrences are demoted into
+ *    `description`; live Messages rejects root combinators and does not document `oneOf`.
  * Tests below that overlap with SDK cases asserting those behaviors are
  * adjusted to our contract; the divergence is called out inline.
  */
@@ -213,12 +215,19 @@ describe("normalizeAnthropicToolSchema — parity with anthropic-sdk-python tran
 	});
 
 	// Mirrors: anthropic-sdk-python/tests/lib/_parse/test_transform.py::test_anyof_schema
-	it("recurses into anyOf variants and spills per-variant constraints", () => {
+	it("recurses into nested anyOf variants and spills per-variant constraints", () => {
 		const out = normalizeAnthropicToolSchema({
-			anyOf: [{ type: "string" }, { type: "integer", minimum: 1 }],
+			type: "object",
+			properties: {
+				value: { anyOf: [{ type: "string" }, { type: "integer", minimum: 1 }] },
+			},
 		});
 		expect(out).toEqual({
-			anyOf: [{ type: "string" }, { type: "integer", description: "{minimum: 1}" }],
+			type: "object",
+			properties: {
+				value: { anyOf: [{ type: "string" }, { type: "integer", description: "{minimum: 1}" }] },
+			},
+			additionalProperties: false,
 		});
 	});
 
@@ -228,23 +237,61 @@ describe("normalizeAnthropicToolSchema — parity with anthropic-sdk-python tran
 		expect(out).toEqual({ type: "string", enum: ["foo", "bar"] });
 	});
 
-	// Mirrors: anthropic-sdk-python/tests/lib/_parse/test_transform.py::test_allof
-	it("recurses into allOf variants and defaults additionalProperties on each object branch", () => {
+	it("spills top-level anyOf variants into description for the Messages API root-combinator boundary", () => {
 		const out = normalizeAnthropicToolSchema({
+			type: "object",
+			properties: { id: { type: "string" } },
+			anyOf: [{ required: ["id"] }, { required: ["name"] }],
+		});
+		expect(out).toEqual({
+			type: "object",
+			properties: { id: { type: "string" } },
+			additionalProperties: false,
+			description: '{anyOf: [{"required":["id"]},{"required":["name"]}]}',
+		});
+	});
+
+	// Divergence: SDK preserves allOf at the schema root, but the live Anthropic Messages
+	// validator rejects root combinators in tool input_schema. Spill it so the model
+	// still sees the constraints.
+	it("spills top-level allOf variants into description instead of sending an unsupported root keyword", () => {
+		const out = normalizeAnthropicToolSchema({
+			type: "object",
+			properties: { id: { type: "string" } },
 			allOf: [
 				{ type: "object", properties: { name: { type: "string" } } },
 				{ type: "object", properties: { age: { type: "integer", minimum: 0 } } },
 			],
 		});
 		expect(out).toEqual({
-			allOf: [
-				{ type: "object", properties: { name: { type: "string" } }, additionalProperties: false },
-				{
-					type: "object",
-					properties: { age: { type: "integer", description: "{minimum: 0}" } },
-					additionalProperties: false,
-				},
-			],
+			type: "object",
+			properties: { id: { type: "string" } },
+			additionalProperties: false,
+			description:
+				'{allOf: [{"type":"object","properties":{"name":{"type":"string"}}},{"type":"object","properties":{"age":{"type":"integer","minimum":0}}}]}',
+		});
+	});
+
+	it("spills oneOf variants at root and nested positions for the same Messages API compatibility boundary", () => {
+		const root = normalizeAnthropicToolSchema({
+			oneOf: [{ type: "string" }, { type: "integer", minimum: 1 }],
+		});
+		expect(root).toEqual({
+			description: '{oneOf: [{"type":"string"},{"type":"integer","minimum":1}]}',
+		});
+
+		const nested = normalizeAnthropicToolSchema({
+			type: "object",
+			properties: {
+				value: { oneOf: [{ type: "string" }, { type: "integer", minimum: 1 }] },
+			},
+		});
+		expect(nested).toEqual({
+			type: "object",
+			properties: {
+				value: { description: '{oneOf: [{"type":"string"},{"type":"integer","minimum":1}]}' },
+			},
+			additionalProperties: false,
 		});
 	});