diff --git a/.github/workflows/agentics-maintenance.yml b/.github/workflows/agentics-maintenance.yml
index 2c1975dfab9..608a43249f0 100644
--- a/.github/workflows/agentics-maintenance.yml
+++ b/.github/workflows/agentics-maintenance.yml
@@ -23,13 +23,13 @@
 #
 # This file defines the generated agentic maintenance workflow for this repository.
 # It runs scheduled cleanup for expiring safe outputs and supports manual maintenance operations.
-#
+# 
 # This workflow is generated automatically when workflows use expiring safe outputs
 # or when repository maintenance features are enabled in .github/workflows/aw.json.
-#
+# 
 # To disable maintenance workflow generation, set in .github/workflows/aw.json:
 # {"maintenance": false}
-#
+# 
 # Agentic maintenance docs:
 # https://github.github.com/gh-aw/reference/ephemerals/#manual-maintenance-operations
 #
diff --git a/actions/setup/js/mcp_server_core.cjs b/actions/setup/js/mcp_server_core.cjs
index 7b707a21c4e..35cd0c84905 100644
--- a/actions/setup/js/mcp_server_core.cjs
+++ b/actions/setup/js/mcp_server_core.cjs
@@ -984,7 +984,11 @@ async function handleMessage(server, req, defaultHandler) {
       server.replyError(id, -32601, `Method not found: ${method}`);
     }
   } catch (e) {
-    server.replyError(id, -32603, e instanceof Error ? e.message : String(e));
+    // Use the error code only if it's a valid JSON-RPC error code (must be a negative integer).
+    // Subprocess exit codes (positive integers like 1, 2, etc.) must not be used as JSON-RPC
+    // error codes, as that would produce non-conformant responses (e.g. "code=1").
+    const code = e && typeof e === "object" && Number.isInteger(e.code) && e.code < 0 ? e.code : -32603;
+    server.replyError(id, code, e && e.message ? String(e.message) : "Internal error");
   }
 }
 
diff --git a/actions/setup/js/mcp_server_core.test.cjs b/actions/setup/js/mcp_server_core.test.cjs
index e5819d785fc..fb27feedc55 100644
--- a/actions/setup/js/mcp_server_core.test.cjs
+++ b/actions/setup/js/mcp_server_core.test.cjs
@@ -471,6 +471,127 @@ describe("mcp_server_core.cjs", () => {
       expect(results).toHaveLength(1);
       expect(results[0].result.content[0].text).toBe("default handler for no_handler_tool");
     });
+
+    it("should return the code and message from a thrown plain object (not Error instance)", async () => {
+      const { registerTool, handleMessage } = await import("./mcp_server_core.cjs");
+
+      registerTool(server, {
+        name: "plain_throw_tool",
+        description: "A tool that throws a plain object",
+        inputSchema: { type: "object", properties: { input: { type: "string" } }, required: ["input"] },
+        handler: () => {
+          throw { code: -32602, message: "ERR_VALIDATION: body required" };
+        },
+      });
+
+      await handleMessage(server, {
+        jsonrpc: "2.0",
+        id: 99,
+        method: "tools/call",
+        params: { name: "plain_throw_tool", arguments: { input: "x" } },
+      });
+
+      expect(results).toHaveLength(1);
+      expect(results[0].error.code).toBe(-32602);
+      expect(results[0].error.message).toBe("ERR_VALIDATION: body required");
+      expect(results[0].error.message).not.toContain("[object Object]");
+    });
+
+    it("should fall back to -32603 when thrown plain object has no valid error code", async () => {
+      const { registerTool, handleMessage } = await import("./mcp_server_core.cjs");
+
+      registerTool(server, {
+        name: "no_code_throw_tool",
+        description: "A tool that throws a plain object without a code",
+        inputSchema: { type: "object", properties: { input: { type: "string" } }, required: ["input"] },
+        handler: () => {
+          throw { message: "something went wrong" };
+        },
+      });
+
+      await handleMessage(server, {
+        jsonrpc: "2.0",
+        id: 100,
+        method: "tools/call",
+        params: { name: "no_code_throw_tool", arguments: { input: "x" } },
+      });
+
+      expect(results).toHaveLength(1);
+      expect(results[0].error.code).toBe(-32603);
+      expect(results[0].error.message).toBe("something went wrong");
+    });
+
+    it("should fall back to -32603 when thrown plain object has a positive code", async () => {
+      const { registerTool, handleMessage } = await import("./mcp_server_core.cjs");
+
+      registerTool(server, {
+        name: "positive_code_throw_tool",
+        description: "A tool that throws a plain object with a positive code",
+        inputSchema: { type: "object", properties: { input: { type: "string" } }, required: ["input"] },
+        handler: () => {
+          throw { code: 1, message: "process exited with code 1" };
+        },
+      });
+
+      await handleMessage(server, {
+        jsonrpc: "2.0",
+        id: 101,
+        method: "tools/call",
+        params: { name: "positive_code_throw_tool", arguments: { input: "x" } },
+      });
+
+      expect(results).toHaveLength(1);
+      expect(results[0].error.code).toBe(-32603);
+      expect(results[0].error.message).toBe("process exited with code 1");
+    });
+
+    it("should use Internal error when thrown plain object has no message property", async () => {
+      const { registerTool, handleMessage } = await import("./mcp_server_core.cjs");
+
+      registerTool(server, {
+        name: "no_message_throw_tool",
+        description: "A tool that throws a plain object without a message",
+        inputSchema: { type: "object", properties: { input: { type: "string" } }, required: ["input"] },
+        handler: () => {
+          throw { code: -32602 };
+        },
+      });
+
+      await handleMessage(server, {
+        jsonrpc: "2.0",
+        id: 102,
+        method: "tools/call",
+        params: { name: "no_message_throw_tool", arguments: { input: "x" } },
+      });
+
+      expect(results).toHaveLength(1);
+      expect(results[0].error.code).toBe(-32602);
+      expect(results[0].error.message).toBe("Internal error");
+    });
+
+    it("should fall back to -32603 when thrown plain object has a non-integer negative code", async () => {
+      const { registerTool, handleMessage } = await import("./mcp_server_core.cjs");
+
+      registerTool(server, {
+        name: "non_integer_code_throw_tool",
+        description: "A tool that throws a plain object with a non-integer code",
+        inputSchema: { type: "object", properties: { input: { type: "string" } }, required: ["input"] },
+        handler: () => {
+          throw { code: -1.5, message: "fractional code should be ignored" };
+        },
+      });
+
+      await handleMessage(server, {
+        jsonrpc: "2.0",
+        id: 103,
+        method: "tools/call",
+        params: { name: "non_integer_code_throw_tool", arguments: { input: "x" } },
+      });
+
+      expect(results).toHaveLength(1);
+      expect(results[0].error.code).toBe(-32603);
+      expect(results[0].error.message).toBe("fractional code should be ignored");
+    });
   });
 
   describe("handleRequest", () => {