diff --git a/packages/ai/src/protocols/anthropic-messages.ts b/packages/ai/src/protocols/anthropic-messages.ts
index 33805d8e2831..cbaca5c1b684 100644
--- a/packages/ai/src/protocols/anthropic-messages.ts
+++ b/packages/ai/src/protocols/anthropic-messages.ts
@@ -997,6 +997,11 @@ const claudeVersion = (id: string) => {
   return { family: match.family, major: Number(match.major), minor: Number(match.minor ?? 0) }
 }
 
+const isClaudeOpus55 = (model: LLMRequest["model"]) => {
+  const version = claudeVersion(model.id)
+  return version?.family === "opus" && version.major === 5 && version.minor === 5
+}
+
 const supportsThinkingBlockBinding = (model: LLMRequest["model"]) => {
   const override = model.compatibility?.supportsThinkingBlockBinding
   if (override !== undefined) return override
@@ -1037,6 +1042,13 @@ const fitThinking = (thinking: AnthropicThinking | undefined, maxTokens: number)
 
 const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (request: LLMRequest) {
   const options = yield* decodeOptions(request.providerOptions ?? {})
+  const opus55 = isClaudeOpus55(request.model)
+  // Neither a manual thinking budget nor forced tool use has an equivalent on Opus 5.5.
+  // Reject explicitly requested behavior rather than silently changing its meaning.
+  if (opus55 && options.thinking && options.thinking.type !== "adaptive")
+    return yield* invalid(
+      "Claude Opus 5.5 only supports adaptive thinking; omit thinking or use effort to control its depth",
+    )
   const management = options.contextManagement
   const outputConfig = options.output_config ?? options.outputConfig
   const format = outputConfig?.format ?? undefined
@@ -1049,6 +1061,8 @@ const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (reques
   const flattened = ProviderShared.flattenToolRequest(updates.request)
   const tools = flattened.tools.length === 0 ? undefined : flattened.tools.map((tool) => lowerTool(breakpoints, tool))
   // Anthropic rejects tool_choice when tools are absent; "none" is only meaningful with tools present.
+  if (tools && opus55 && (request.toolChoice?.type === "required" || request.toolChoice?.type === "tool"))
+    return yield* invalid("Claude Opus 5.5 does not support forced tool use; use toolChoice auto or none")
   const toolChoice = tools === undefined || !request.toolChoice ? undefined : yield* lowerToolChoice(request.toolChoice)
   const systemParts = request.system.filter((part) => part.text.length > 0)
   const system =
diff --git a/packages/ai/test/provider/anthropic-messages.test.ts b/packages/ai/test/provider/anthropic-messages.test.ts
index 173a8c7b8f98..085d1b13af66 100644
--- a/packages/ai/test/provider/anthropic-messages.test.ts
+++ b/packages/ai/test/provider/anthropic-messages.test.ts
@@ -170,6 +170,41 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("rejects unsupported thinking modes for Claude Opus 5.5 before sending", () =>
+    Effect.gen(function* () {
+      for (const thinking of [{ type: "disabled" as const }, { type: "enabled" as const, budgetTokens: 2_048 }]) {
+        const error = yield* compileRequest(
+          LLMRequest.update(request, {
+            model: AnthropicMessages.route.model({ id: "claude-opus-5-5" }),
+            providerOptions: { thinking },
+          }),
+        ).pipe(Effect.flip)
+
+        expect(error.reason._tag).toBe("InvalidRequest")
+        expect(error.message).toContain("Claude Opus 5.5")
+        expect(error.message).toContain("effort")
+      }
+    }),
+  )
+
+  it.effect("keeps adaptive thinking and effort on Claude Opus 5.5", () =>
+    Effect.gen(function* () {
+      const prepared = yield* compileRequest(
+        LLMRequest.update(request, {
+          model: AnthropicMessages.route.model({ id: "claude-opus-5-5" }),
+          providerOptions: { thinking: { type: "adaptive", display: "summarized" }, effort: "low" },
+        }),
+      )
+
+      expect(prepared.body.thinking).toEqual({
+        type: "adaptive",
+        display: "summarized",
+        block_binding: { prefix_mismatch_behavior: "drop_block" },
+      })
+      expect(prepared.body.output_config).toEqual({ effort: "low" })
+    }),
+  )
+
   it.effect("fits the thinking budget to half the output limit", () =>
     Effect.gen(function* () {
       const thinking = (maxTokens: number) =>
@@ -730,6 +765,49 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("rejects forced tool use for Claude Opus 5.5 before sending", () =>
+    Effect.gen(function* () {
+      for (const toolChoice of ["required" as const, { type: "tool" as const, name: "lookup" }]) {
+        const error = yield* compileRequest(
+          LLM.request({
+            model: AnthropicMessages.route.model({ id: "anthropic.claude-opus-5-5@default" }),
+            prompt: "Look it up",
+            tools: [{ name: "lookup", description: "Look things up", inputSchema: { type: "object" } }],
+            toolChoice,
+          }),
+        ).pipe(Effect.flip)
+
+        expect(error.reason._tag).toBe("InvalidRequest")
+        expect(error.message).toContain("Claude Opus 5.5")
+        expect(error.message).toContain("auto")
+      }
+    }),
+  )
+
+  it.effect("preserves supported tool choices on Opus 5.5 and forced choices on Opus 5", () =>
+    Effect.gen(function* () {
+      const choices = yield* Effect.forEach(
+        [
+          { id: "claude-opus-5-5", toolChoice: "auto" as const },
+          { id: "claude-opus-5-5", toolChoice: "none" as const },
+          { id: "claude-opus-5", toolChoice: "required" as const },
+          { id: "claude-opus-5", toolChoice: { type: "tool" as const, name: "lookup" } },
+        ],
+        ({ id, toolChoice }) =>
+          compileRequest(
+            LLM.request({
+              model: AnthropicMessages.route.model({ id }),
+              prompt: "Look it up",
+              tools: [{ name: "lookup", description: "Look things up", inputSchema: { type: "object" } }],
+              toolChoice,
+            }),
+          ).pipe(Effect.map((prepared) => prepared.body.tool_choice)),
+      )
+
+      expect(choices).toEqual([{ type: "auto" }, { type: "none" }, { type: "any" }, { type: "tool", name: "lookup" }])
+    }),
+  )
+
   // Regression: read tool results must stay structured so base64 media data is
   // not JSON-stringified into `tool_result.content`.
   it.effect("lowers media tool-result content as structured blocks", () =>
diff --git a/packages/ai/test/provider/anthropic-thinking-binding.test.ts b/packages/ai/test/provider/anthropic-thinking-binding.test.ts
index 3df8e873b774..f7826062e0e7 100644
--- a/packages/ai/test/provider/anthropic-thinking-binding.test.ts
+++ b/packages/ai/test/provider/anthropic-thinking-binding.test.ts
@@ -12,6 +12,7 @@ for (const [id, enabled] of [
   ["anthropic/claude-fable-5.1", true],
   ["claude-fable-5-1@default", true],
   ["claude-fable-5-1@20260901", true],
+  ["claude-opus-5-5", true],
   ["claude-sonnet-6", true],
   ["claude-opus-5-20260901", false],
   ["claude-opus-4-8", false],