Sanitize malformed tool-call arguments before echoing history

scripts/monitor-pilot/live-runner.ts

@@ -282,7 +282,20 @@ function parseArgs(call: { readonly id: string; readonly function: { readonly na
 }
 
 function toolCallMessage(response: LlmResponse): ChatMessage {
-	return response.message
+	const toolCalls = response.message.tool_calls?.map((call) => {
+		if (isJsonObjectArguments(call.function.arguments)) return call
+		return { ...call, function: { ...call.function, arguments: "{}" } }
+	})
+	return { ...response.message, tool_calls: toolCalls }
+}
+
+function isJsonObjectArguments(raw: string): boolean {
+	try {
+		const parsed: unknown = JSON.parse(raw)
+		return parsed !== null && typeof parsed === "object" && !Array.isArray(parsed)
+	} catch {
+		return false
+	}
 }
 
 function stableCallBatchSignature(calls: readonly PilotToolCall[]): string {

src/monitoring/__tests__/live-runner.test.ts

@@ -456,6 +456,25 @@ async function testReasoningHeavyNativeOverrunIsAccepted(): Promise<void> {
 	} finally { await fs.rm(directory, { recursive: true, force: true }) }
 }
 
+async function testMalformedArgumentsAreSanitizedOnEcho(): Promise<void> {
+	const directory = await scratch()
+	try {
+		const broken = { name: "apply_diff", args: "{\"content\":\"truncated" }
+		const done = { name: "attempt_completion",
+			args: JSON.stringify({ result: "done" }) }
+		const client = new MockBillingClient([broken, done])
+		const summary = await collectLiveDevelopment({ outputDir: directory, client })
+		assert.equal(summary.abortedAttempts, 0)
+		assert.equal(summary.completedRuns, 32)
+		const history = client.requests[1]?.messages ?? []
+		const calls = history.filter((message) => {
+			return (message.tool_calls?.length ?? 0) > 0
+		})
+		const echoed = calls[0]?.tool_calls?.[0]?.function.arguments
+		assert.equal(echoed, "{}")
+	} finally { await fs.rm(directory, { recursive: true, force: true }) }
+}
+
 async function testHeldoutCannotStartWithoutQualifiedDevelopment(): Promise<void> {
 	const directory = await scratch()
 	try {
@@ -537,6 +556,7 @@ await testCopiedGlobalLedgerEpochsDoNotBlockResume()
 await testKnownPerRunAdmissionStopCompletesAndSettlesTrajectory()
 await testCompletionOverrunIsJournaledBeforeAnyToolDispatch()
 await testReasoningHeavyNativeOverrunIsAccepted()
+await testMalformedArgumentsAreSanitizedOnEcho()
 await testHeldoutCannotStartWithoutQualifiedDevelopment()
 await testHeldoutRejectsDifferentFrozenOutputCapBeforeInference()
 await testStorageRootSymlinkIsRejectedBeforeInference()