diff --git a/packages/opencode/src/session/tools.ts b/packages/opencode/src/session/tools.ts
index 0f401c7562fa..c0756ea891b1 100644
--- a/packages/opencode/src/session/tools.ts
+++ b/packages/opencode/src/session/tools.ts
@@ -99,6 +99,7 @@ export const resolve = Effect.fn("SessionTools.resolve")(function* (input: {
     tools[item.id] = tool({
       description: item.description,
       inputSchema: jsonSchema(schema),
+      ...(item.strict !== undefined && { strict: item.strict }),
       execute(args, options) {
         return run.promise(
           Effect.gen(function* () {
diff --git a/packages/opencode/src/tool/registry.ts b/packages/opencode/src/tool/registry.ts
index 15acc757f3d4..1aa1d2b25cf5 100644
--- a/packages/opencode/src/tool/registry.ts
+++ b/packages/opencode/src/tool/registry.ts
@@ -134,6 +134,7 @@ const layer = Layer.effect(
             id,
             parameters,
             jsonSchema,
+            strict: def.strict,
             description: def.description,
             execute: (args, toolCtx) =>
               Effect.gen(function* () {
@@ -309,6 +310,7 @@ const layer = Layer.effect(
             description: tool.description,
             parameters: tool.parameters,
             jsonSchema: tool.jsonSchema,
+            strict: tool.strict,
           }
           yield* plugin.trigger("tool.definition", { toolID: tool.id }, output)
           const jsonSchema =
@@ -326,6 +328,7 @@ const layer = Layer.effect(
               .join("\n"),
             parameters: output.parameters,
             jsonSchema,
+            strict: output.strict,
             execute: tool.execute,
             formatValidationError: tool.formatValidationError,
           }
diff --git a/packages/opencode/src/tool/tool.ts b/packages/opencode/src/tool/tool.ts
index e5e7802858ca..fee05e00a65c 100644
--- a/packages/opencode/src/tool/tool.ts
+++ b/packages/opencode/src/tool/tool.ts
@@ -60,6 +60,7 @@ export interface Def<
   description: string
   parameters: Parameters
   jsonSchema?: JSONSchema7
+  strict?: boolean
   execute(args: Schema.Schema.Type<Parameters>, ctx: Context): Effect.Effect<ExecuteResult<M>>
   formatValidationError?(error: unknown): string
 }
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 0b88fa59dff8..5e45a528a750 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -586,6 +586,67 @@ describe("ProviderTransform.options - gpt-5 textVerbosity", () => {
     expect(result.tools.lookup.strict).toBe(false)
   })
 
+  test("a tool's strict flag survives on bedrock and is still overridden on mantle", async () => {
+    const prepare = (npm: string, apiId: string, url: string) =>
+      Effect.runPromise(
+        LLMRequestPrep.prepare({
+          user: {
+            id: "msg_user-test",
+            sessionID,
+            role: "user",
+            time: { created: Date.now() },
+            agent: "test",
+            model: { providerID: "amazon-bedrock", modelID: apiId },
+          } as any,
+          sessionID,
+          model: {
+            ...createGpt5Model(apiId),
+            id: `amazon-bedrock/${apiId}`,
+            providerID: "amazon-bedrock",
+            api: { id: apiId, url, npm },
+          },
+          agent: {
+            name: "test",
+            mode: "primary",
+            options: {},
+            permission: [],
+          } as any,
+          system: [],
+          messages: [{ role: "user", content: "Hello" }],
+          tools: {
+            lookup: {
+              description: "Look up a value",
+              inputSchema: jsonSchema({ type: "object", properties: {} }),
+              strict: true,
+            },
+          },
+          provider: { id: "amazon-bedrock", options: {} } as any,
+          auth: undefined,
+          plugin: {
+            trigger: (_name: string, _input: unknown, output: unknown) => Effect.succeed(output),
+            list: () => Effect.succeed([]),
+            init: () => Effect.void,
+          } as any,
+          flags: { outputTokenMax: 32_000, client: "test" } as any,
+          isWorkflow: false,
+        }),
+      )
+
+    const converse = await prepare(
+      "@ai-sdk/amazon-bedrock",
+      "anthropic.claude-sonnet-4-6",
+      "https://bedrock-runtime.us-east-1.amazonaws.com",
+    )
+    const mantle = await prepare(
+      "@ai-sdk/amazon-bedrock/mantle",
+      "openai.gpt-5.5",
+      "https://bedrock-mantle.us-east-2.api.aws/openai/v1",
+    )
+
+    expect(converse.tools.lookup.strict).toBe(true)
+    expect(mantle.tools.lookup.strict).toBe(false)
+  })
+
   test("gpt-5.1 should have textVerbosity set to low", () => {
     const model = createGpt5Model("gpt-5.1")
     const result = ProviderTransform.options({ model, sessionID, providerOptions: {} })
diff --git a/packages/opencode/test/tool/registry.test.ts b/packages/opencode/test/tool/registry.test.ts
index c8c5fac59559..6bf2b983880a 100644
--- a/packages/opencode/test/tool/registry.test.ts
+++ b/packages/opencode/test/tool/registry.test.ts
@@ -194,6 +194,39 @@ describe("tool.registry", () => {
     }),
   )
 
+  it.instance("carries strict from a custom tool definition", () =>
+    Effect.gen(function* () {
+      const test = yield* TestInstance
+      const tool = path.join(test.directory, ".opencode", "tool")
+      yield* Effect.promise(() => fs.mkdir(tool, { recursive: true }))
+      yield* Effect.promise(() =>
+        Bun.write(
+          path.join(tool, "strict_tool.ts"),
+          [
+            "export default {",
+            "  description: 'strict tool',",
+            "  args: {},",
+            "  strict: true,",
+            "  execute: async () => 'ok',",
+            "}",
+            "",
+          ].join("\n"),
+        ),
+      )
+
+      const registry = yield* ToolRegistry.Service
+      const agents = yield* Agent.Service
+      const tools = yield* registry.tools({
+        providerID: ProviderV2.ID.opencode,
+        modelID: ModelV2.ID.make("test"),
+        agent: yield* agents.defaultInfo(),
+      })
+
+      expect(tools.find((item) => item.id === "strict_tool")?.strict).toBe(true)
+      expect(tools.filter((item) => item.id !== "strict_tool").every((item) => item.strict === undefined)).toBe(true)
+    }),
+  )
+
   it.instance("ignores non-tool exports in .opencode/tool files", () =>
     Effect.gen(function* () {
       const test = yield* TestInstance
diff --git a/packages/plugin/src/index.ts b/packages/plugin/src/index.ts
index edfa0139dfca..efdcdfd42da6 100644
--- a/packages/plugin/src/index.ts
+++ b/packages/plugin/src/index.ts
@@ -331,5 +331,8 @@ export interface Hooks {
   /**
    * Modify tool definitions (description and parameters) sent to LLM
    */
-  "tool.definition"?: (input: { toolID: string }, output: { description: string; parameters: any }) => Promise<void>
+  "tool.definition"?: (
+    input: { toolID: string },
+    output: { description: string; parameters: any; strict?: boolean },
+  ) => Promise<void>
 }
diff --git a/packages/plugin/src/tool.ts b/packages/plugin/src/tool.ts
index 9c6daa34d04a..ca3349fab5ee 100644
--- a/packages/plugin/src/tool.ts
+++ b/packages/plugin/src/tool.ts
@@ -45,6 +45,7 @@ export type ToolResult =
 export function tool<Args extends z.ZodRawShape>(input: {
   description: string
   args: Args
+  strict?: boolean
   execute(args: z.infer<z.ZodObject<Args>>, context: ToolContext): Promise<ToolResult>
 }) {
   return input