fix: normalize MCP tool schemas missing properties field for OpenAI Responses API

Tools with no parameters produce { type: "object" } schemas without a
properties field. The OpenAI Responses API rejects these, silently
crashing entire sessions.

Add properties: {} injection in normalizeToolParameters() and
convertTools() to ensure all object-type schemas include a properties
field.

Closes #58246

src/agents/openai-ws-message-conversion.ts

@@ -276,12 +276,21 @@ export function convertTools(tools: Context["tools"]): FunctionToolDefinition[]
   if (!tools || tools.length === 0) {
     return [];
   }
-  return tools.map((tool) => ({
-    type: "function" as const,
-    name: tool.name,
-    description: typeof tool.description === "string" ? tool.description : undefined,
-    parameters: (tool.parameters ?? {}) as Record<string, unknown>,
-  }));
+  return tools.map((tool) => {
+    const params = (tool.parameters ?? {}) as Record<string, unknown>;
+    // Ensure `type: "object"` schemas include `properties` — the OpenAI Responses
+    // API rejects bare `{ type: "object" }` from MCP tools with no parameters.
+    const normalizedParams =
+      params.type === "object" && !("properties" in params)
+        ? { ...params, properties: {} }
+        : params;
+    return {
+      type: "function" as const,
+      name: tool.name,
+      description: typeof tool.description === "string" ? tool.description : undefined,
+      parameters: normalizedParams,
+    };
+  });
 }
 
 export function planTurnInput(params: {

src/agents/openai-ws-stream.test.ts

@@ -354,6 +354,35 @@ describe("convertTools", () => {
     const result = convertTools(tools as Parameters<typeof convertTools>[0]);
     expect(result[0]?.name).toBe("ping");
   });
+
+  it("injects properties:{} for type:object schemas missing properties (MCP no-param tools)", () => {
+    const tools = [
+      { name: "list_regions", description: "List AWS regions", parameters: { type: "object" } },
+    ];
+    const result = convertTools(tools as unknown as Parameters<typeof convertTools>[0]);
+    expect(result).toHaveLength(1);
+    expect(result[0]).toMatchObject({
+      type: "function",
+      name: "list_regions",
+      description: "List AWS regions",
+      parameters: { type: "object", properties: {} },
+    });
+  });
+
+  it("preserves existing properties on type:object schemas", () => {
+    const tools = [
+      {
+        name: "exec",
+        description: "Run a command",
+        parameters: { type: "object", properties: { cmd: { type: "string" } } },
+      },
+    ];
+    const result = convertTools(tools as unknown as Parameters<typeof convertTools>[0]);
+    expect(result[0]?.parameters).toEqual({
+      type: "object",
+      properties: { cmd: { type: "string" } },
+    });
+  });
 });
 
 // ─────────────────────────────────────────────────────────────────────────────

src/agents/pi-tools.schema.test.ts

@@ -4,6 +4,55 @@ import { normalizeToolParameters } from "./pi-tools.schema.js";
 import type { AnyAgentTool } from "./pi-tools.types.js";
 
 describe("normalizeToolParameters", () => {
+  it("injects properties:{} for type:object schemas missing properties (MCP no-param tools)", () => {
+    const tool: AnyAgentTool = {
+      name: "list_regions",
+      label: "list_regions",
+      description: "List all AWS regions",
+      parameters: { type: "object" },
+      execute: vi.fn(),
+    };
+
+    const normalized = normalizeToolParameters(tool);
+
+    const parameters = normalized.parameters as Record<string, unknown>;
+    expect(parameters.type).toBe("object");
+    expect(parameters.properties).toEqual({});
+  });
+
+  it("preserves existing properties on type:object schemas", () => {
+    const tool: AnyAgentTool = {
+      name: "query",
+      label: "query",
+      description: "Run a query",
+      parameters: { type: "object", properties: { q: { type: "string" } } },
+      execute: vi.fn(),
+    };
+
+    const normalized = normalizeToolParameters(tool);
+
+    const parameters = normalized.parameters as Record<string, unknown>;
+    expect(parameters.type).toBe("object");
+    expect(parameters.properties).toEqual({ q: { type: "string" } });
+  });
+
+  it("injects properties:{} for type:object with only additionalProperties", () => {
+    const tool: AnyAgentTool = {
+      name: "passthrough",
+      label: "passthrough",
+      description: "Accept any input",
+      parameters: { type: "object", additionalProperties: true },
+      execute: vi.fn(),
+    };
+
+    const normalized = normalizeToolParameters(tool);
+
+    const parameters = normalized.parameters as Record<string, unknown>;
+    expect(parameters.type).toBe("object");
+    expect(parameters.properties).toEqual({});
+    expect(parameters.additionalProperties).toBe(true);
+  });
+
   it("strips compat-declared unsupported schema keywords without provider-specific branching", () => {
     const tool: AnyAgentTool = {
       name: "demo",

src/agents/pi-tools.schema.ts

@@ -133,6 +133,22 @@ export function normalizeToolParameters(
     });
   }
 
+  // MCP tools with no parameters produce `{ type: "object" }` without `properties`.
+  // The OpenAI function-calling API rejects bare `{ type: "object" }` schemas.
+  // Inject an empty `properties` object — semantically identical in JSON Schema.
+  if (
+    "type" in schema &&
+    !("properties" in schema) &&
+    !Array.isArray(schema.anyOf) &&
+    !Array.isArray(schema.oneOf)
+  ) {
+    const schemaWithProperties = { ...schema, properties: {} };
+    return preserveToolMeta({
+      ...tool,
+      parameters: applyProviderCleaning(schemaWithProperties),
+    });
+  }
+
   const variantKey = Array.isArray(schema.anyOf)
     ? "anyOf"
     : Array.isArray(schema.oneOf)