diff --git a/.changeset/fix-tool-output-provider-metadata.md b/.changeset/fix-tool-output-provider-metadata.md
new file mode 100644
index 000000000..88f20eaa1
--- /dev/null
+++ b/.changeset/fix-tool-output-provider-metadata.md
@@ -0,0 +1,14 @@
+---
+"@voltagent/core": patch
+---
+
+fix(core): forward providerMetadata on tool-result and tool-error stream chunks
+
+Google Vertex thinking models attach `providerMetadata` (containing `thoughtSignature`) to
+tool-output stream events. The `tool-result` → `tool-output-available` and `tool-error` →
+`tool-output-error` conversions in `convertFullStreamChunkToUIMessageStream` were not forwarding
+this field, causing the AI SDK's UI message stream schema validation to reject the chunk as
+having unrecognized keys. This broke all tool calls when using `@ai-sdk/google-vertex` with
+thinking models (e.g. `gemini-3-flash-preview`).
+
+Fixes #1195
diff --git a/packages/core/src/agent/streaming/guardrail-stream.spec.ts b/packages/core/src/agent/streaming/guardrail-stream.spec.ts
index 58eed50a3..a9c600cbe 100644
--- a/packages/core/src/agent/streaming/guardrail-stream.spec.ts
+++ b/packages/core/src/agent/streaming/guardrail-stream.spec.ts
@@ -402,4 +402,110 @@ describe("Output guardrail streaming integration", () => {
     }
     expect(collected.join("")).toBe("hello");
   });
+
+  it("preserves providerMetadata on tool-result events (Google Vertex thinking models)", async () => {
+    const googleThoughtSignature = "CiQBjz1rXwFvT1I/B/3qqqGc2FdAgzW+FJY4Tg/zPaWUtF6imFw=";
+    const parts: VoltAgentTextStreamPart[] = [
+      { type: "start" } as VoltAgentTextStreamPart,
+      {
+        type: "tool-call",
+        toolCallId: "tool-1",
+        toolName: "getWeather",
+        input: { location: "Perth" },
+        providerMetadata: { google: { thoughtSignature: googleThoughtSignature } },
+      } as VoltAgentTextStreamPart,
+      {
+        type: "tool-result",
+        toolCallId: "tool-1",
+        output: { weather: { location: "Perth", condition: "Sunny", temperature: 25 } },
+        providerMetadata: { google: { thoughtSignature: googleThoughtSignature } },
+      } as VoltAgentTextStreamPart,
+      { type: "text-start", id: "text-1" } as VoltAgentTextStreamPart,
+      { type: "text-delta", id: "text-1", delta: "The weather is sunny." } as any,
+      { type: "text-end", id: "text-1" } as VoltAgentTextStreamPart,
+      { type: "finish", finishReason: "stop" } as any,
+    ];
+
+    const pipeline = buildPipeline(parts, {
+      id: "passthrough",
+      name: "Passthrough",
+      handler: async (_ctx) => ({ pass: true }) as const,
+      streamHandler: ({ part }) => part,
+    });
+
+    const emitted: VoltAgentTextStreamPart[] = [];
+    for await (const chunk of pipeline.fullStream) {
+      emitted.push(chunk as VoltAgentTextStreamPart);
+    }
+
+    await pipeline.finalizePromise;
+
+    // tool-call should preserve providerMetadata
+    const toolCall = emitted.find((c) => c.type === "tool-call") as any;
+    expect(toolCall).toBeDefined();
+    expect(toolCall.providerMetadata).toEqual({
+      google: { thoughtSignature: googleThoughtSignature },
+    });
+
+    // tool-result should preserve providerMetadata (the fix for #1195)
+    const toolResult = emitted.find((c) => c.type === "tool-result") as any;
+    expect(toolResult).toBeDefined();
+    expect(toolResult.providerMetadata).toEqual({
+      google: { thoughtSignature: googleThoughtSignature },
+    });
+  });
+
+  it("forwards providerMetadata to tool-output-available in UI stream (fixes #1195)", async () => {
+    const googleThoughtSignature = "CiQBjz1rXwFvT1I/B/3qqqGc2FdAgzW+FJY4Tg/zPaWUtF6imFw=";
+    const parts: VoltAgentTextStreamPart[] = [
+      { type: "start" } as VoltAgentTextStreamPart,
+      {
+        type: "tool-call",
+        toolCallId: "tool-1",
+        toolName: "getWeather",
+        input: { location: "Perth" },
+        providerMetadata: { google: { thoughtSignature: googleThoughtSignature } },
+      } as VoltAgentTextStreamPart,
+      {
+        type: "tool-result",
+        toolCallId: "tool-1",
+        output: { weather: { location: "Perth", condition: "Sunny", temperature: 25 } },
+        providerMetadata: { google: { thoughtSignature: googleThoughtSignature } },
+      } as VoltAgentTextStreamPart,
+      { type: "text-start", id: "text-1" } as VoltAgentTextStreamPart,
+      { type: "text-delta", id: "text-1", delta: "The weather is sunny." } as any,
+      { type: "text-end", id: "text-1" } as VoltAgentTextStreamPart,
+      { type: "finish", finishReason: "stop" } as any,
+    ];
+
+    const pipeline = buildPipeline(parts, {
+      id: "passthrough",
+      name: "Passthrough",
+      handler: async (_ctx) => ({ pass: true }) as const,
+      streamHandler: ({ part }) => part,
+    });
+
+    const uiStream = pipeline.createUIStream();
+    const uiChunks: Array<Record<string, unknown>> = [];
+    for await (const chunk of uiStream) {
+      uiChunks.push(chunk as Record<string, unknown>);
+    }
+
+    await pipeline.finalizePromise;
+
+    // tool-input-available (from tool-call) should have providerMetadata
+    const toolInput = uiChunks.find((c) => c.type === "tool-input-available") as any;
+    expect(toolInput).toBeDefined();
+    expect(toolInput.providerMetadata).toEqual({
+      google: { thoughtSignature: googleThoughtSignature },
+    });
+
+    // tool-output-available (from tool-result) should have providerMetadata
+    // Before the fix, this was missing and caused zod schema validation to fail
+    const toolOutput = uiChunks.find((c) => c.type === "tool-output-available") as any;
+    expect(toolOutput).toBeDefined();
+    expect(toolOutput.providerMetadata).toEqual({
+      google: { thoughtSignature: googleThoughtSignature },
+    });
+  });
 });
diff --git a/packages/core/src/agent/streaming/guardrail-stream.ts b/packages/core/src/agent/streaming/guardrail-stream.ts
index 9d88e4fb2..049d156ea 100644
--- a/packages/core/src/agent/streaming/guardrail-stream.ts
+++ b/packages/core/src/agent/streaming/guardrail-stream.ts
@@ -573,6 +573,7 @@ function convertFullStreamChunkToUIMessageStream<UI_MESSAGE extends UIMessage>({
         toolCallId?: string;
         output?: unknown;
         providerExecuted?: boolean;
+        providerMetadata?: unknown;
         dynamic?: unknown;
       };
       return {
@@ -580,6 +581,7 @@ function convertFullStreamChunkToUIMessageStream<UI_MESSAGE extends UIMessage>({
         toolCallId: typed.toolCallId,
         output: typed.output,
         ...(typed.providerExecuted != null ? { providerExecuted: typed.providerExecuted } : {}),
+        ...(typed.providerMetadata != null ? { providerMetadata: typed.providerMetadata } : {}),
         ...(typed.dynamic != null ? { dynamic: typed.dynamic } : {}),
       } as InferUIMessageChunk<UI_MESSAGE>;
     }
@@ -589,6 +591,7 @@ function convertFullStreamChunkToUIMessageStream<UI_MESSAGE extends UIMessage>({
         toolCallId?: string;
         error?: unknown;
         providerExecuted?: boolean;
+        providerMetadata?: unknown;
         dynamic?: unknown;
       };
       return {
@@ -596,16 +599,18 @@ function convertFullStreamChunkToUIMessageStream<UI_MESSAGE extends UIMessage>({
         toolCallId: typed.toolCallId,
         errorText: onError(typed.error),
         ...(typed.providerExecuted != null ? { providerExecuted: typed.providerExecuted } : {}),
+        ...(typed.providerMetadata != null ? { providerMetadata: typed.providerMetadata } : {}),
         ...(typed.dynamic != null ? { dynamic: typed.dynamic } : {}),
       } as InferUIMessageChunk<UI_MESSAGE>;
     }
 
     case "tool-output": {
-      const typed = part as { toolCallId?: string; output: unknown };
+      const typed = part as { toolCallId?: string; output: unknown; providerMetadata?: unknown };
       return {
         type: "tool-output-available",
         toolCallId: typed.toolCallId,
         output: typed.output,
+        ...(typed.providerMetadata != null ? { providerMetadata: typed.providerMetadata } : {}),
       } as InferUIMessageChunk<UI_MESSAGE>;
     }