Q81: unreadable tool arguments are a named protocol error, never an empty call

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/agent-run-loop.test.ts

@@ -286,6 +286,18 @@ describe("agent run loop (real tool dispatch)", () => {
     const results = (lastToolMessage as { results: Array<{ output: string }> }).results;
     expect(results.filter((result) => result.output.startsWith("ERROR: not run: this turn asked for 20 tool calls; the limit is 16"))).toHaveLength(4);
 
+    // A call whose arguments could not be read is answered with the reason and never run.
+    const broken = [{ text: "Writing.", toolCalls: [{ id: "c_bad", name: "send_message", arguments: {}, argumentsError: "not valid JSON (Unexpected end of JSON input): {\"body\":\"hi" }] }];
+    let brokenResults: Array<{ output: string }> = [];
+    const breaking = fakeProvider(broken, connection.db);
+    const brokenRun = await runAgentLoop({ providers: { ...breaking, runToolTurn: async (input: { messages: Array<{ role: string; results?: Array<{ output: string }> }> }) => {
+      const last = input.messages.at(-1);
+      if (last?.role === "tool") brokenResults = last.results ?? [];
+      return breaking.runToolTurn();
+    } }, tools } as never, loopInput);
+    expect(brokenRun.toolCallCount).toBe(0);
+    expect(brokenResults.map((result) => result.output)).toEqual(['ERROR: not run: the arguments of send_message were not valid JSON (Unexpected end of JSON input): {"body":"hi. Call send_message again with a complete JSON object.']);
+
     // A third goal audit in one session is refused before it runs.
     const audits = [{ text: "Closing.", toolCalls: [call("complete_goal", { evidence: "a" }), call("complete_goal", { evidence: "b" }), call("complete_goal", { evidence: "c" })] }];
     let auditResults: Array<{ output: string }> = [];

tests/unit/tool-call-arguments.test.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test } from "vitest";
+import { parseChatChoice, accumulateChatStream } from "../../src/main/services/provider-tool-turns/chat-format";
+import { parseResponsesOutput } from "../../src/main/services/provider-tool-turns/responses-format";
+import { parseAnthropicContent } from "../../src/main/services/provider-tool-turns/anthropic-format";
+import { parseBedrockOutput } from "../../src/main/services/provider-tool-turns/bedrock-format";
+import { parseGeminiResponse } from "../../src/main/services/provider-tool-turns/gemini-format";
+import type { JsonRecord, ProviderEventStreamMessage } from "../../src/main/services/provider-http";
+
+async function* stream(items: JsonRecord[]): AsyncIterable<ProviderEventStreamMessage> {
+  for (const data of items) yield { eventName: null, data, rawData: JSON.stringify(data) };
+}
+
+const chat = (args: unknown) => parseChatChoice({ choices: [{ message: { content: "", tool_calls: [{ id: "c1", function: { name: "write_file", arguments: args } }] }, finish_reason: "tool_calls" }] }).toolCalls[0];
+
+describe("tool-call arguments", () => {
+  test("half-written JSON is a named protocol error that keeps the parser's reason and the raw text", () => {
+    expect(chat('{"path":"a.txt","content":"he')).toMatchObject({ arguments: {}, argumentsError: expect.stringMatching(/^not valid JSON \(.+\): \{"path":"a\.txt","content":"he$/) });
+  });
+
+  test("JSON that is not an object is an error naming what it was", () => {
+    expect(chat("[1,2]").argumentsError).toBe("expected a JSON object, got an array");
+    expect(chat('"a.txt"').argumentsError).toBe("expected a JSON object, got a string");
+    expect(parseAnthropicContent({ stop_reason: "tool_use", content: [{ type: "tool_use", id: "t", name: "f", input: "x" }] }).toolCalls[0].argumentsError).toBe("expected a JSON object, got a string");
+    expect(parseBedrockOutput({ stopReason: "tool_use", output: { message: { content: [{ toolUse: { toolUseId: "t", name: "f", input: [] } }] } } }).toolCalls[0].argumentsError).toBe("expected a JSON object, got an array");
+    expect(parseGeminiResponse({ candidates: [{ content: { parts: [{ functionCall: { name: "f", args: 3 } }] }, finishReason: "STOP" }] }).toolCalls[0].argumentsError).toBe("expected a JSON object, got a number");
+  });
+
+  test("absent or empty arguments are a call with none; objects pass through", async () => {
+    expect(chat("")).toEqual({ id: "c1", name: "write_file", arguments: {} });
+    expect(chat('{"path":"a"}')).toEqual({ id: "c1", name: "write_file", arguments: { path: "a" } });
+    expect(parseResponsesOutput({ status: "completed", output: [{ type: "function_call", call_id: "c2", name: "f", arguments: "{}" }] }).toolCalls[0]).toEqual({ id: "c2", name: "f", arguments: {} });
+    const streamed = await accumulateChatStream(stream([
+      { choices: [{ delta: { tool_calls: [{ index: 0, id: "c3", function: { name: "f", arguments: '{"a":' } }] }, finish_reason: "tool_calls" }] },
+    ]));
+    expect(streamed.toolCalls[0].argumentsError).toMatch(/^not valid JSON/);
+    const typed = await accumulateChatStream(stream([
+      { choices: [{ delta: { tool_calls: [{ index: 0, id: "c4", function: { name: "f", arguments: 42 } }, { index: 1, id: "c5", function: { name: "g", arguments: { a: 1 } } }] }, finish_reason: "tool_calls" }] },
+    ]));
+    const mixed = await accumulateChatStream(stream([
+      { choices: [{ delta: { tool_calls: [{ index: 0, id: "c6", function: { name: "f", arguments: 42 } }] } }] },
+      { choices: [{ delta: { tool_calls: [{ index: 0, function: { arguments: {} } }] } }] },
+      { choices: [{ delta: { tool_calls: [{ index: 1, id: "c7", function: { name: "g", arguments: '{"a":' } }] } }] },
+      { choices: [{ delta: { tool_calls: [{ index: 1, function: { arguments: { a: 1 } } }] }, finish_reason: "tool_calls" }] },
+    ]));
+    expect(mixed.toolCalls.map((call) => call.argumentsError)).toEqual([
+      "expected one JSON object, got 2 non-text fragments", "expected one JSON object, got text and 1 non-text fragment",
+    ]);
+    expect(typed.toolCalls).toEqual([{ id: "c4", name: "f", arguments: {}, argumentsError: "expected a JSON object, got a number" }, { id: "c5", name: "g", arguments: { a: 1 } }]);
+  });
+});