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fix: include schema guidance in empty-arguments error to prevent missing-tool reports #40542

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pelikhan merged 3 commits into from
Jun 21, 2026
+44 −2
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fix: include schema guidance in empty-arguments error to prevent missing-tool reports #40542

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Copilot Jun 21, 2026
af41caf
Fix: include schema guidance in empty-args error to prevent missing t…
Copilot Jun 21, 2026
4601b90
test: add handleRequest probe-detection test for empty-args schema gu…
Copilot Jun 21, 2026
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6 changes: 4 additions & 2 deletions actions/setup/js/mcp_server_core.cjs
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Original file line number Diff line number Diff line change
Expand Up @@ -772,9 +772,10 @@ async function handleRequest(server, request, defaultHandler) {
if (missing.length) {
const hasRequiredFields = tool.inputSchema && Array.isArray(tool.inputSchema.required) && tool.inputSchema.required.length > 0;
if (hasRequiredFields && Object.keys(args).length === 0) {
const schemaGuidance = generateEnhancedErrorMessage(tool.inputSchema.required, name, tool.inputSchema);

@github-actions github-actions Bot Jun 21, 2026

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[/diagnose] Minor consistency nit: in the empty-args branch missing already equals tool.inputSchema.required (all fields are missing), so passing tool.inputSchema.required to generateEnhancedErrorMessage is equivalent but slightly diverges from the partial-args path just below (line 783) which uses missing.

💡 Suggested change 
// Before
const schemaGuidance = generateEnhancedErrorMessage(tool.inputSchema.required, name, tool.inputSchema);

// After (matches the partial-args path)
const schemaGuidance = generateEnhancedErrorMessage(missing, name, tool.inputSchema);

Same change applies to line 945 in the handleMessage path. This makes both branches read consistently and avoids the impression that a different field list is intentional.

throw {
code: -32602,
message: `Empty arguments are not allowed — this tool is write-once, not a discovery probe. To inspect the schema, use the tools/list MCP method. To signal that no action is needed, call \`noop\` with a \`message\`.`,
message: `Empty arguments are not allowed — this tool is write-once, not a discovery probe. To inspect the schema, use the tools/list MCP method. To signal that no action is needed, call \`noop\` with a \`message\`.\n\n${schemaGuidance}`,
};
}
throw {
Expand Down Expand Up @@ -941,10 +942,11 @@ async function handleMessage(server, req, defaultHandler) {
if (missing.length) {
const hasRequiredFields = tool.inputSchema && Array.isArray(tool.inputSchema.required) && tool.inputSchema.required.length > 0;
if (hasRequiredFields && Object.keys(args).length === 0) {
const schemaGuidance = generateEnhancedErrorMessage(tool.inputSchema.required, name, tool.inputSchema);
server.replyError(
id,
-32602,
`Empty arguments are not allowed — this tool is write-once, not a discovery probe. To inspect the schema, use the tools/list MCP method. To signal that no action is needed, call \`noop\` with a \`message\`.`
`Empty arguments are not allowed — this tool is write-once, not a discovery probe. To inspect the schema, use the tools/list MCP method. To signal that no action is needed, call \`noop\` with a \`message\`.\n\n${schemaGuidance}`
);
return;
}
Expand Down
40 changes: 40 additions & 0 deletions actions/setup/js/mcp_server_core.test.cjs
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Original file line number Diff line number Diff line change
Expand Up @@ -354,6 +354,9 @@ describe("mcp_server_core.cjs", () => {
expect(results[0].error.message).toContain("write-once, not a discovery probe");
expect(results[0].error.message).toContain("tools/list");
expect(results[0].error.message).toContain("noop");
// Schema guidance should be included so the model can retry without calling tools/list
expect(results[0].error.message).toContain("Example:");
expect(results[0].error.message).toContain("Required parameter");
Comment on lines +357 to +359

@github-actions github-actions Bot Jun 21, 2026

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[/tdd] These new assertions cover only the handleMessage probe-detection path. The handleRequest describe block (line 476) has no equivalent probe-detection test — a future regression there would go undetected.

💡 Suggested addition in the handleRequest describe block 
it("should return probe-detection error with schema guidance for empty arguments", async () => {
  const { createServer, registerTool, handleRequest } = await import("./mcp_server_core.cjs");
  const server = createServer({ name: "safe-outputs", version: "1.0.0" });

  registerTool(server, {
    name: "create_issue",
    description: "Create an issue",
    inputSchema: {
      type: "object",
      properties: {
        title: { type: "string", description: "Issue title" },
        body:  { type: "string", description: "Issue body" },
      },
      required: ["title", "body"],
    },
    handler: () => ({ content: [] }),
  });

  const response = await handleRequest(server, {
    jsonrpc: "2.0", id: 10, method: "tools/call",
    params: { name: "create_issue", arguments: {} },
  });

  expect(response.error.code).toBe(-32602);
  expect(response.error.message).toContain("write-once, not a discovery probe");
  expect(response.error.message).toContain("Required parameter");
  expect(response.error.message).toContain("Example:");
});

});

it("should return enhanced error for partially-supplied but invalid required fields", async () => {
Expand Down Expand Up @@ -568,6 +571,43 @@ describe("mcp_server_core.cjs", () => {
expect(response.error.message).toContain("too short");
expect(response.error.message).toContain("20");
});

it("should return error for empty arguments object (probe detection) via handleRequest", async () => {
const { createServer, registerTool, handleRequest } = await import("./mcp_server_core.cjs");
const server = createServer({ name: "test-server", version: "1.0.0" });

registerTool(server, {
name: "probe_tool",
description: "A tool to test probe detection",
inputSchema: {
type: "object",
properties: { input: { type: "string", description: "Some input" } },
required: ["input"],
},
handler: args => ({
content: [{ type: "text", text: `received: ${args.input}` }],
}),
});

const response = await handleRequest(server, {
jsonrpc: "2.0",
id: 10,
method: "tools/call",
params: {
name: "probe_tool",
arguments: {}, // completely empty — probe attempt
},
});

expect(response.error).toBeDefined();
expect(response.error.code).toBe(-32602);
expect(response.error.message).toContain("write-once, not a discovery probe");
expect(response.error.message).toContain("tools/list");
expect(response.error.message).toContain("noop");
// Schema guidance should be included so the model can retry without calling tools/list
expect(response.error.message).toContain("Example:");
expect(response.error.message).toContain("Required parameter");
});
});

describe("loadToolHandlers", () => {
Expand Down
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