diff --git a/.changeset/responses-tool-strictness.md b/.changeset/responses-tool-strictness.md
new file mode 100644
index 00000000000..5828d25de3c
--- /dev/null
+++ b/.changeset/responses-tool-strictness.md
@@ -0,0 +1,5 @@
+---
+"@opencode-ai/core": patch
+---
+
+Preserve explicit function tool strictness in Copilot Responses requests.
diff --git a/packages/core/src/github-copilot/responses/openai-responses-prepare-tools.ts b/packages/core/src/github-copilot/responses/openai-responses-prepare-tools.ts
index 2592636dded..f87c53aa1a1 100644
--- a/packages/core/src/github-copilot/responses/openai-responses-prepare-tools.ts
+++ b/packages/core/src/github-copilot/responses/openai-responses-prepare-tools.ts
@@ -47,7 +47,7 @@ export function prepareResponsesTools({
           name: tool.name,
           description: tool.description,
           parameters: tool.inputSchema,
-          strict: strictJsonSchema,
+          strict: tool.strict ?? strictJsonSchema,
         })
         break
       case "provider": {
diff --git a/packages/core/test/github-copilot/openai-responses-prepare-tools.test.ts b/packages/core/test/github-copilot/openai-responses-prepare-tools.test.ts
new file mode 100644
index 00000000000..011a538a0bb
--- /dev/null
+++ b/packages/core/test/github-copilot/openai-responses-prepare-tools.test.ts
@@ -0,0 +1,20 @@
+import { expect, test } from "bun:test"
+import type { LanguageModelV3FunctionTool } from "@ai-sdk/provider"
+import { prepareResponsesTools } from "@opencode-ai/core/github-copilot/responses/openai-responses-prepare-tools"
+
+function prepare(strict: boolean | undefined, strictJsonSchema: boolean) {
+  const tool: LanguageModelV3FunctionTool = {
+    type: "function",
+    name: "lookup",
+    inputSchema: { type: "object", properties: {} },
+    strict,
+  }
+  return prepareResponsesTools({ tools: [tool], strictJsonSchema }).tools?.[0]
+}
+
+test("function tools prefer explicit strictness over the global fallback", () => {
+  expect(prepare(true, false)).toMatchObject({ type: "function", strict: true })
+  expect(prepare(false, true)).toMatchObject({ type: "function", strict: false })
+  expect(prepare(undefined, true)).toMatchObject({ type: "function", strict: true })
+  expect(prepare(undefined, false)).toMatchObject({ type: "function", strict: false })
+})