diff --git a/.changeset/fix-bedrock-thinking-generate-object.md b/.changeset/fix-bedrock-thinking-generate-object.md
new file mode 100644
index 00000000000..04143c2fd7b
--- /dev/null
+++ b/.changeset/fix-bedrock-thinking-generate-object.md
@@ -0,0 +1,5 @@
+---
+"@effect/ai-amazon-bedrock": patch
+---
+
+`generateObject` no longer fails when extended thinking is configured via `withConfigOverride`. Anthropic's API rejects requests that set `thinking` in `additionalModelRequestFields` alongside a forced `toolChoice` — which `generateObject` always does. The fix strips `thinking` from `additionalModelRequestFields` in the `json` response format path before the request is sent.
diff --git a/packages/ai/amazon-bedrock/src/AmazonBedrockLanguageModel.ts b/packages/ai/amazon-bedrock/src/AmazonBedrockLanguageModel.ts
index dbcadd08643..729c05a76fd 100644
--- a/packages/ai/amazon-bedrock/src/AmazonBedrockLanguageModel.ts
+++ b/packages/ai/amazon-bedrock/src/AmazonBedrockLanguageModel.ts
@@ -206,6 +206,26 @@ export const make = Effect.fnUntraced(function*(options: {
       const { messages, system } = yield* prepareMessages(providerOptions)
       const { additionalTools, betas, toolConfig } = yield* prepareTools(providerOptions, config)
       const responseFormat = providerOptions.responseFormat
+
+      // Anthropic rejects requests that combine extended thinking with forced tool use.
+      // generateObject always forces toolChoice, so strip "thinking" in the json path.
+      // Return an explicit object (even if empty) when fields are present so the spread
+      // overrides the thinking config already placed in the request by ...config above.
+      const requestAdditionalFields: Record<string, unknown> | undefined = responseFormat.type === "json"
+        ? (Predicate.isNotUndefined(config.additionalModelRequestFields) ||
+            Predicate.isNotUndefined(additionalTools))
+          ? (() => {
+            const { thinking: _thinking, ...rest } = {
+              ...config.additionalModelRequestFields,
+              ...additionalTools
+            }
+            return rest
+          })()
+          : undefined
+        : Predicate.isNotUndefined(additionalTools)
+        ? { ...config.additionalModelRequestFields, ...additionalTools }
+        : undefined
+
       const request: typeof ConverseRequest.Encoded = {
         ...config,
         system,
@@ -231,13 +251,8 @@ export const make = Effect.fnUntraced(function*(options: {
           ? { toolConfig }
           : {}),
         // Handle additional model request fields
-        ...(Predicate.isNotUndefined(additionalTools)
-          ? {
-            additionalModelRequestFields: {
-              ...config.additionalModelRequestFields,
-              ...additionalTools
-            }
-          }
+        ...(Predicate.isNotUndefined(requestAdditionalFields)
+          ? { additionalModelRequestFields: requestAdditionalFields }
           : {})
       }
       return { betas, request }
diff --git a/packages/ai/amazon-bedrock/test/AmazonBedrockLanguageModel.test.ts b/packages/ai/amazon-bedrock/test/AmazonBedrockLanguageModel.test.ts
new file mode 100644
index 00000000000..f6026113efa
--- /dev/null
+++ b/packages/ai/amazon-bedrock/test/AmazonBedrockLanguageModel.test.ts
@@ -0,0 +1,175 @@
+import * as LanguageModel from "@effect/ai/LanguageModel"
+import { assert, describe, it } from "@effect/vitest"
+import * as Effect from "effect/Effect"
+import * as Layer from "effect/Layer"
+import * as Schema from "effect/Schema"
+import { AmazonBedrockClient } from "../src/AmazonBedrockClient.js"
+import * as AmazonBedrockLanguageModel from "../src/AmazonBedrockLanguageModel.js"
+import { ConverseResponse } from "../src/AmazonBedrockSchema.js"
+import type { ConverseRequest } from "../src/AmazonBedrockSchema.js"
+
+// ---------------------------------------------------------------------------
+// Schema decoders
+// ---------------------------------------------------------------------------
+
+const decodeConverseResponse = Schema.decodeUnknownSync(ConverseResponse)
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+const TestModel = "us.anthropic.claude-3-5-sonnet-20241022-v2:0" as const
+const TestObjectName = "MockResult"
+const MockResultSchema = Schema.Struct({ field: Schema.String })
+
+const textResponse = () =>
+  decodeConverseResponse({
+    output: {
+      message: {
+        role: "assistant",
+        content: [{ text: "hello" }]
+      }
+    },
+    metrics: { latencyMs: 100 },
+    stopReason: "end_turn",
+    usage: { inputTokens: 10, outputTokens: 5, totalTokens: 15 }
+  })
+
+const toolUseResponse = (name: string) =>
+  decodeConverseResponse({
+    output: {
+      message: {
+        role: "assistant",
+        content: [{
+          toolUse: {
+            toolUseId: "test-id",
+            name,
+            input: { field: "value" }
+          }
+        }]
+      }
+    },
+    metrics: { latencyMs: 100 },
+    stopReason: "tool_use",
+    usage: { inputTokens: 10, outputTokens: 5, totalTokens: 15 }
+  })
+
+const makeCapturingLayer = (
+  captured: Array<typeof ConverseRequest.Encoded>,
+  response: ConverseResponse
+) =>
+  AmazonBedrockLanguageModel.layer({ model: TestModel }).pipe(
+    Layer.provide(
+      Layer.succeed(AmazonBedrockClient, {
+        client: null as any,
+        streamRequest: null as any,
+        converse: (opts) =>
+          Effect.sync(() => {
+            captured.push(opts.payload)
+            return response
+          }),
+        converseStream: null as any
+      })
+    )
+  )
+
+// ---------------------------------------------------------------------------
+// makeRequest — request construction tests
+// ---------------------------------------------------------------------------
+
+describe("AmazonBedrockLanguageModel", () => {
+  describe("makeRequest / additionalModelRequestFields", () => {
+    it.effect("strips thinking when using generateObject (forced toolChoice.tool)", () =>
+      Effect.gen(function*() {
+        const captured: Array<typeof ConverseRequest.Encoded> = []
+
+        yield* LanguageModel.generateObject({
+          prompt: [],
+          schema: MockResultSchema,
+          objectName: TestObjectName
+        }).pipe(
+          AmazonBedrockLanguageModel.withConfigOverride({
+            additionalModelRequestFields: {
+              thinking: { type: "enabled", budget_tokens: 5000 }
+            }
+          }),
+          Effect.provide(makeCapturingLayer(captured, toolUseResponse(TestObjectName)))
+        )
+
+        assert.strictEqual(captured.length, 1)
+        const req = captured[0]
+
+        // Must force tool use for generateObject
+        assert.deepStrictEqual(req.toolConfig?.toolChoice, { tool: { name: TestObjectName } })
+
+        // Anthropic rejects thinking + forced tool use — must be stripped
+        assert.isUndefined(req.additionalModelRequestFields?.["thinking"])
+      }))
+
+    it.effect("preserves thinking when using generateText (no forced toolChoice)", () =>
+      Effect.gen(function*() {
+        const captured: Array<typeof ConverseRequest.Encoded> = []
+
+        yield* LanguageModel.generateText({ prompt: [] }).pipe(
+          AmazonBedrockLanguageModel.withConfigOverride({
+            additionalModelRequestFields: {
+              thinking: { type: "enabled", budget_tokens: 5000 }
+            }
+          }),
+          Effect.provide(makeCapturingLayer(captured, textResponse()))
+        )
+
+        assert.strictEqual(captured.length, 1)
+        const req = captured[0]
+
+        // thinking must flow through for non-forced-tool-use requests
+        assert.deepStrictEqual(req.additionalModelRequestFields?.["thinking"], {
+          type: "enabled",
+          budget_tokens: 5000
+        })
+      }))
+
+    it.effect("does not set additionalModelRequestFields when none configured and using generateObject", () =>
+      Effect.gen(function*() {
+        const captured: Array<typeof ConverseRequest.Encoded> = []
+
+        yield* LanguageModel.generateObject({
+          prompt: [],
+          schema: MockResultSchema,
+          objectName: TestObjectName
+        }).pipe(
+          Effect.provide(makeCapturingLayer(captured, toolUseResponse(TestObjectName)))
+        )
+
+        assert.strictEqual(captured.length, 1)
+        // No additionalModelRequestFields should be injected when none was configured
+        assert.isUndefined(captured[0].additionalModelRequestFields)
+      }))
+
+    it.effect("preserves non-thinking fields in additionalModelRequestFields with generateObject", () =>
+      Effect.gen(function*() {
+        const captured: Array<typeof ConverseRequest.Encoded> = []
+
+        yield* LanguageModel.generateObject({
+          prompt: [],
+          schema: MockResultSchema,
+          objectName: TestObjectName
+        }).pipe(
+          AmazonBedrockLanguageModel.withConfigOverride({
+            additionalModelRequestFields: {
+              thinking: { type: "enabled", budget_tokens: 5000 },
+              someOtherField: "preserved"
+            }
+          }),
+          Effect.provide(makeCapturingLayer(captured, toolUseResponse(TestObjectName)))
+        )
+
+        assert.strictEqual(captured.length, 1)
+        const req = captured[0]
+
+        // thinking stripped, other fields survive
+        assert.isUndefined(req.additionalModelRequestFields?.["thinking"])
+        assert.strictEqual(req.additionalModelRequestFields?.["someOtherField"], "preserved")
+      }))
+  })
+})