diff --git a/packages/llm/src/protocols/openai-chat.ts b/packages/llm/src/protocols/openai-chat.ts
index 9ac85b07b139..62a56c1a05ad 100644
--- a/packages/llm/src/protocols/openai-chat.ts
+++ b/packages/llm/src/protocols/openai-chat.ts
@@ -53,6 +53,9 @@ const OpenAIChatAssistantToolCall = Schema.Struct({
     name: Schema.String,
     arguments: Schema.String,
   }),
+  extra_content: Schema.optional(
+    Schema.Struct({ google: Schema.Struct({ thought_signature: Schema.optional(Schema.String) }) }),
+  ),
 })
 type OpenAIChatAssistantToolCall = Schema.Schema.Type<typeof OpenAIChatAssistantToolCall>
 
@@ -139,6 +142,9 @@ const OpenAIChatToolCallDelta = Schema.Struct({
   index: Schema.Number,
   id: optionalNull(Schema.String),
   function: optionalNull(OpenAIChatToolCallDeltaFunction),
+  extra_content: optionalNull(
+    Schema.Struct({ google: Schema.Struct({ thought_signature: optionalNull(Schema.String) }) }),
+  ),
 })
 type OpenAIChatToolCallDelta = Schema.Schema.Type<typeof OpenAIChatToolCallDelta>
 
@@ -193,6 +199,13 @@ const lowerToolChoice = (toolChoice: NonNullable<LLMRequest["toolChoice"]>) =>
     tool: (name) => ({ type: "function" as const, function: { name } }),
   })
 
+const thoughtSignature = (part: ToolCallPart) => {
+  const google = part.providerMetadata?.google
+  return ProviderShared.isRecord(google) && typeof google.thoughtSignature === "string"
+    ? google.thoughtSignature
+    : undefined
+}
+
 const lowerToolCall = (part: ToolCallPart): OpenAIChatAssistantToolCall => ({
   id: part.id,
   type: "function",
@@ -200,6 +213,9 @@ const lowerToolCall = (part: ToolCallPart): OpenAIChatAssistantToolCall => ({
     name: part.name,
     arguments: ProviderShared.encodeJson(part.input),
   },
+  extra_content: thoughtSignature(part)
+    ? { google: { thought_signature: thoughtSignature(part) } }
+    : undefined,
 })
 
 const lowerMedia = Effect.fn("OpenAIChat.lowerMedia")(function* (part: MediaPart) {
@@ -431,7 +447,14 @@ const step = (state: ParserState, event: OpenAIChatEvent) =>
         ADAPTER,
         tools,
         tool.index,
-        { id: tool.id ?? undefined, name: tool.function?.name ?? undefined, text: tool.function?.arguments ?? "" },
+        {
+          id: tool.id ?? undefined,
+          name: tool.function?.name ?? undefined,
+          text: tool.function?.arguments ?? "",
+          providerMetadata: tool.extra_content?.google?.thought_signature
+            ? { google: { thoughtSignature: tool.extra_content.google.thought_signature } }
+            : undefined,
+        },
         "OpenAI Chat tool call delta is missing id or name",
       )
       if (ToolStream.isError(result)) return yield* result
diff --git a/packages/llm/src/protocols/utils/tool-stream.ts b/packages/llm/src/protocols/utils/tool-stream.ts
index 8e07a64bfed8..59ab2e9df7de 100644
--- a/packages/llm/src/protocols/utils/tool-stream.ts
+++ b/packages/llm/src/protocols/utils/tool-stream.ts
@@ -118,7 +118,12 @@ export const appendOrStart = <K extends StreamKey>(
   route: string,
   tools: State<K>,
   key: K,
-  delta: { readonly id?: string; readonly name?: string; readonly text: string },
+  delta: {
+    readonly id?: string
+    readonly name?: string
+    readonly text: string
+    readonly providerMetadata?: ProviderMetadata
+  },
   missingToolMessage: string,
 ): AppendOutcome<K> | LLMError => {
   const current = tools[key]
@@ -131,7 +136,7 @@ export const appendOrStart = <K extends StreamKey>(
     name,
     input: `${current?.input ?? ""}${delta.text}`,
     providerExecuted: current?.providerExecuted,
-    providerMetadata: current?.providerMetadata,
+    providerMetadata: delta.providerMetadata ?? current?.providerMetadata,
   }
   if (current && delta.text.length === 0 && current.id === id && current.name === name)
     return { tools, tool: current, events: [] }
diff --git a/packages/llm/test/provider/openai-chat.test.ts b/packages/llm/test/provider/openai-chat.test.ts
index b736dc9dd33c..6df75af2cf07 100644
--- a/packages/llm/test/provider/openai-chat.test.ts
+++ b/packages/llm/test/provider/openai-chat.test.ts
@@ -552,14 +552,52 @@ describe("OpenAI Chat route", () => {
     }),
   )
 
+  it.effect("round-trips Google OpenAI-compatible tool thought signatures", () =>
+    Effect.gen(function* () {
+      const call = ToolCallPart.make({
+        id: "call_1",
+        name: "lookup",
+        input: { query: "weather" },
+        providerMetadata: { google: { thoughtSignature: "tool_sig" } },
+      })
+      const prepared = yield* LLMClient.prepare<OpenAIChat.OpenAIChatBody>(
+        LLM.request({
+          model,
+          messages: [Message.user("Check the weather."), Message.assistant([call])],
+          tools: [{ name: "lookup", description: "Lookup data", inputSchema: { type: "object" } }],
+        }),
+      )
+
+      expect(prepared.body.messages[1]).toEqual({
+        role: "assistant",
+        content: null,
+        tool_calls: [
+          {
+            id: "call_1",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"weather"}' },
+            extra_content: { google: { thought_signature: "tool_sig" } },
+          },
+        ],
+      })
+    }),
+  )
+
   it.effect("assembles streamed tool call input", () =>
     Effect.gen(function* () {
       const body = sseEvents(
         deltaChunk({
           role: "assistant",
-          tool_calls: [{ index: 0, id: "call_1", function: { name: "lookup", arguments: '{"query"' } }],
+          tool_calls: [
+            {
+              index: 0,
+              id: "call_1",
+              function: { name: "lookup", arguments: '{"query"' },
+            },
+          ],
         }),
         deltaChunk({ tool_calls: [{ index: 0, function: { arguments: ':"weather"}' } }] }),
+        deltaChunk({ tool_calls: [{ index: 0, extra_content: { google: { thought_signature: "tool_sig_late" } } }] }),
         deltaChunk({}, "tool_calls"),
       )
       const response = yield* LLMClient.generate(
@@ -568,19 +606,20 @@ describe("OpenAI Chat route", () => {
         }),
       ).pipe(Effect.provide(fixedResponse(body)))
 
+      const signatureMetadata = { google: { thoughtSignature: "tool_sig_late" } }
       expect(response.events).toEqual([
         { type: "step-start", index: 0 },
-        { type: "tool-input-start", id: "call_1", name: "lookup", providerMetadata: undefined },
+        { type: "tool-input-start", id: "call_1", name: "lookup", providerMetadata: signatureMetadata },
         { type: "tool-input-delta", id: "call_1", name: "lookup", text: '{"query"' },
         { type: "tool-input-delta", id: "call_1", name: "lookup", text: ':"weather"}' },
-        { type: "tool-input-end", id: "call_1", name: "lookup", providerMetadata: undefined },
+        { type: "tool-input-end", id: "call_1", name: "lookup", providerMetadata: signatureMetadata },
         {
           type: "tool-call",
           id: "call_1",
           name: "lookup",
           input: { query: "weather" },
           providerExecuted: undefined,
-          providerMetadata: undefined,
+          providerMetadata: signatureMetadata,
         },
         { type: "step-finish", index: 0, reason: "tool-calls", usage: undefined, providerMetadata: undefined },
         { type: "finish", reason: "tool-calls", usage: undefined },