fix: suppress raw JSON parse errors from leaking to Discord channels (#59076) [AI-assisted]

When streaming tool calls with long CJK text, the Anthropic SDK's SSE
parser can throw SyntaxError (e.g. 'Expected "," or "}" after property
value in JSON at position 334'). This error propagates through pi-ai's
catch block as an assistant error message and reaches formatAssistantErrorText,
which did not recognize the pattern and forwarded the raw error text to
the Discord channel.

- Add isStreamingJsonParseError() detector in pi-embedded-helpers/errors.ts
- Intercept in formatAssistantErrorText() to return a friendly message
- Also guard sanitizeUserFacingText() errorContext path for defense-in-depth
- Add 5 test cases covering various JSON parse error patterns and contexts

Verified: 131 related tests pass, pnpm build succeeds, zero regression.
AI-assisted: code fix and test generation verified by automated test runs

src/agents/pi-embedded-helpers.formatassistanterrortext.test.ts

@@ -7,6 +7,7 @@ import {
   getApiErrorPayloadFingerprint,
   formatRawAssistantErrorForUi,
   isRawApiErrorPayload,
+  sanitizeUserFacingText,
 } from "./pi-embedded-helpers.js";
 import { makeAssistantMessageFixture } from "./test-helpers/assistant-message-fixtures.js";
 
@@ -349,6 +350,40 @@ describe("formatAssistantErrorText", () => {
       "LLM request failed: provider returned an invalid streaming response. Please try again.",
     );
   });
+
+  it("sanitizes streaming JSON parse errors from Anthropic SDK (#59076)", () => {
+    const msg = makeAssistantError(
+      "Expected ',' or '}' after property value in JSON at position 334 (line 1 column 335)",
+    );
+    expect(formatAssistantErrorText(msg)).toBe(
+      "LLM streaming response contained a malformed fragment. Please try again.",
+    );
+  });
+
+  it("sanitizes 'Expected double-quoted property name' JSON parse errors (#59076)", () => {
+    const msg = makeAssistantError(
+      "Expected double-quoted property name in JSON at position 8912 (line 219 column 5)",
+    );
+    expect(formatAssistantErrorText(msg)).toBe(
+      "LLM streaming response contained a malformed fragment. Please try again.",
+    );
+  });
+
+  it("sanitizes 'Unexpected token' JSON parse errors (#59076)", () => {
+    const msg = makeAssistantError("Unexpected token < in JSON at position 0");
+    expect(formatAssistantErrorText(msg)).toBe(
+      "LLM streaming response contained a malformed fragment. Please try again.",
+    );
+  });
+
+  it("keeps provider request-validation JSON diagnostics actionable", () => {
+    const msg = makeAssistantError(
+      '{"type":"error","error":{"type":"invalid_request_error","message":"Expected value in JSON at position 12 for messages.0.content"}}',
+    );
+    expect(formatAssistantErrorText(msg)).toBe(
+      "LLM request rejected: Expected value in JSON at position 12 for messages.0.content",
+    );
+  });
 });
 
 describe("formatRawAssistantErrorForUi", () => {
@@ -424,3 +459,22 @@ describe("raw API error payload helpers", () => {
     );
   });
 });
+
+describe("sanitizeUserFacingText — streaming JSON parse error (#59076)", () => {
+  it("rewrites JSON parse error in error context", () => {
+    const result = sanitizeUserFacingText(
+      "Expected ',' or '}' after property value in JSON at position 334 (line 1 column 335)",
+      { errorContext: true },
+    );
+    expect(result).toBe("LLM streaming response contained a malformed fragment. Please try again.");
+  });
+
+  it("does not rewrite JSON parse error when not in error context", () => {
+    // When not in error context, the text could be legitimate assistant content
+    // mentioning JSON errors. Don't rewrite.
+    const text =
+      "Expected ',' or '}' after property value in JSON at position 334 (line 1 column 335)";
+    const result = sanitizeUserFacingText(text, { errorContext: false });
+    expect(result).toBe(text);
+  });
+});

src/agents/pi-embedded-helpers/errors.ts

@@ -46,6 +46,7 @@ import {
   isInvalidStreamingEventOrderError,
   isLikelyHttpErrorText,
   isRawApiErrorPayload,
+  isStreamingJsonParseError,
   sanitizeUserFacingText,
 } from "./sanitize-user-facing-text.js";
 import type { FailoverReason } from "./types.js";
@@ -1139,6 +1140,10 @@ export function formatAssistantErrorText(
     return formatRawAssistantErrorForUi(raw);
   }
 
+  if (isStreamingJsonParseError(raw)) {
+    return "LLM streaming response contained a malformed fragment. Please try again.";
+  }
+
   // Never return raw unhandled errors - log for debugging but return safe message
   if (raw.length > 600) {
     log.warn(`Long error truncated: ${raw.slice(0, 200)}`);

src/agents/pi-embedded-helpers/sanitize-user-facing-text.ts

@@ -209,6 +209,13 @@ export function isInvalidStreamingEventOrderError(raw: string): boolean {
   );
 }
 
+export function isStreamingJsonParseError(raw: string): boolean {
+  if (!raw) {
+    return false;
+  }
+  return /\b(?:expected|unexpected)\b.+\bin json\b.+\bposition\b/i.test(raw);
+}
+
 function hasRateLimitTpmHint(raw: string): boolean {
   const lower = normalizeLowercaseStringOrEmpty(raw);
   return /\btpm\b/i.test(lower) || lower.includes("tokens per minute");
@@ -419,6 +426,10 @@ export function sanitizeUserFacingText(text: unknown, opts?: { errorContext?: bo
       return formatRawAssistantErrorForUi(trimmed);
     }
 
+    if (isStreamingJsonParseError(trimmed)) {
+      return "LLM streaming response contained a malformed fragment. Please try again.";
+    }
+
     if (ERROR_PREFIX_RE.test(trimmed)) {
       const prefixedCopy = formatRateLimitOrOverloadedErrorCopy(trimmed);
       if (prefixedCopy) {