Summary

Fixes #11649

sanitizeUserFacingText() runs isBillingErrorMessage() against all assistant reply text, not just error payloads. When the assistant naturally discusses billing, credits, or payment topics, the sanitizer false-positives and replaces the entire reply with the canned billing error message.

Root Cause

isBillingErrorMessage() matches broadly on keyword combinations (e.g. "billing" + "credits"/"payment"/"plan"). This is correct for short API error messages, but false-positives on normal assistant replies that happen to discuss those topics.

Fix

Add a length guard (trimmed.length < 500) before the isBillingErrorMessage check in sanitizeUserFacingText. Real API billing errors are short error strings (<500 chars). Normal assistant replies discussing billing topics are longer and should pass through unchanged.

This is the same pattern already used for context overflow detection (shouldRewriteContextOverflowText has conversational-length guards).

Changes

• src/agents/pi-embedded-helpers/errors.ts: Add trimmed.length < 500 guard (1 line)
• Test: Add 2 test cases — short billing errors still caught, long billing discussions pass through
• CHANGELOG.md: Add entry

Verification

Bug reproduced on main branch ❌

Case 1: Short API error "402 Payment Required" (20 chars)
  Caught as billing error: true ✅

Case 2: Short billing combo (58 chars)
  Caught as billing error: true ✅

Case 3: Long assistant reply discussing billing (618 chars)
  Result preserved: false
  False-positive (replaced with billing error): true ❌ BUG

❌ BUG REPRODUCED: Long assistant reply was replaced with billing error message.

A 618-char assistant reply discussing billing/credits/payment was entirely replaced with the canned billing error string.

Fix verified on feature branch ✅

Case 1: Short API error "402 Payment Required" (20 chars)
  Caught as billing error: true ✅

Case 2: Short billing combo (58 chars)
  Caught as billing error: true ✅

Case 3: Long assistant reply discussing billing (618 chars)
  Result preserved: true
  False-positive: false ✅ CORRECT

✅ FIX VERIFIED: Short billing errors caught, long replies pass through.

Unit tests ✅

Test Files  1 passed (1)
     Tests  12 passed (12)

Lint ✅

Found 0 warnings and 0 errors.