diff --git a/src/agents/tools/message-tool.test.ts b/src/agents/tools/message-tool.test.ts
index a289908ac1..5c974e001c 100644
--- a/src/agents/tools/message-tool.test.ts
+++ b/src/agents/tools/message-tool.test.ts
@@ -162,6 +162,80 @@ describe("message tool description", () => {
   });
 });
 
+describe("message tool reasoning tag sanitization", () => {
+  it("strips <think> tags from text field before sending", async () => {
+    mocks.runMessageAction.mockClear();
+    mocks.runMessageAction.mockResolvedValue({
+      kind: "send",
+      action: "send",
+      channel: "signal",
+      to: "signal:+15551234567",
+      handledBy: "plugin",
+      payload: {},
+      dryRun: true,
+    } satisfies MessageActionRunResult);
+
+    const tool = createMessageTool({ config: {} as never });
+
+    await tool.execute("1", {
+      action: "send",
+      target: "signal:+15551234567",
+      text: "<think>internal reasoning</think>Hello!",
+    });
+
+    const call = mocks.runMessageAction.mock.calls[0]?.[0];
+    expect(call?.params?.text).toBe("Hello!");
+  });
+
+  it("strips <think> tags from content field before sending", async () => {
+    mocks.runMessageAction.mockClear();
+    mocks.runMessageAction.mockResolvedValue({
+      kind: "send",
+      action: "send",
+      channel: "discord",
+      to: "discord:123",
+      handledBy: "plugin",
+      payload: {},
+      dryRun: true,
+    } satisfies MessageActionRunResult);
+
+    const tool = createMessageTool({ config: {} as never });
+
+    await tool.execute("1", {
+      action: "send",
+      target: "discord:123",
+      content: "<think>reasoning here</think>Reply text",
+    });
+
+    const call = mocks.runMessageAction.mock.calls[0]?.[0];
+    expect(call?.params?.content).toBe("Reply text");
+  });
+
+  it("passes through text without reasoning tags unchanged", async () => {
+    mocks.runMessageAction.mockClear();
+    mocks.runMessageAction.mockResolvedValue({
+      kind: "send",
+      action: "send",
+      channel: "signal",
+      to: "signal:+15551234567",
+      handledBy: "plugin",
+      payload: {},
+      dryRun: true,
+    } satisfies MessageActionRunResult);
+
+    const tool = createMessageTool({ config: {} as never });
+
+    await tool.execute("1", {
+      action: "send",
+      target: "signal:+15551234567",
+      text: "Normal message without any tags",
+    });
+
+    const call = mocks.runMessageAction.mock.calls[0]?.[0];
+    expect(call?.params?.text).toBe("Normal message without any tags");
+  });
+});
+
 describe("message tool sandbox passthrough", () => {
   it("forwards sandboxRoot to runMessageAction", async () => {
     mocks.runMessageAction.mockClear();
diff --git a/src/agents/tools/message-tool.ts b/src/agents/tools/message-tool.ts
index 08309b2efe..44618cbede 100644
--- a/src/agents/tools/message-tool.ts
+++ b/src/agents/tools/message-tool.ts
@@ -16,6 +16,7 @@ import { GATEWAY_CLIENT_IDS, GATEWAY_CLIENT_MODES } from "../../gateway/protocol
 import { getToolResult, runMessageAction } from "../../infra/outbound/message-action-runner.js";
 import { normalizeTargetForProvider } from "../../infra/outbound/target-normalization.js";
 import { normalizeAccountId } from "../../routing/session-key.js";
+import { stripReasoningTagsFromText } from "../../shared/text/reasoning-tags.js";
 import { normalizeMessageChannel } from "../../utils/message-channel.js";
 import { resolveSessionAgentId } from "../agent-scope.js";
 import { listChannelSupportedActions } from "../channel-tools.js";
@@ -405,7 +406,17 @@ export function createMessageTool(options?: MessageToolOptions): AnyAgentTool {
         err.name = "AbortError";
         throw err;
       }
-      const params = args as Record<string, unknown>;
+      // Shallow-copy so we don't mutate the original event args (used for logging/dedup).
+      const params = { ...(args as Record<string, unknown>) };
+
+      // Strip reasoning tags from text fields — models may include <think>…</think>
+      // in tool arguments, and the messaging tool send path has no other tag filtering.
+      for (const field of ["text", "content", "message", "caption"]) {
+        if (typeof params[field] === "string") {
+          params[field] = stripReasoningTagsFromText(params[field] as string);
+        }
+      }
+
       const cfg = options?.config ?? loadConfig();
       const action = readStringParam(params, "action", {
         required: true,