[pcbeingused333]

Problem

@posthog/ai's Anthropic streaming wrapper only reads output_tokens (and the web search count) from message_delta. Anthropic's message_delta usage is cumulative: with server tools (web search, web fetch, code execution) its input_tokens, cache_creation_input_tokens and cache_read_input_tokens grow past the message_start values during the response. Streamed generations therefore under-report $ai_input_tokens and the cache counts.

This is the same bug that was fixed for posthog-python in PostHog/posthog-python#1074.

Changes

In packages/ai/src/anthropic/index.ts, the message_delta usage now also updates inputTokens, cacheCreationInputTokens and cacheReadInputTokens, keeping the larger of the message_start value and the delta value (totals never shrink). A field the delta omits or under-reports stays at the message_start value. rawUsage merging is unchanged.

A patch changeset for @posthog/ai is included.

Test strategy

• Unit (packages/ai/tests/anthropic.test.ts, new PostHogAnthropic - streaming usage block, no API key needed): one test where message_delta reports larger cumulative input/cache counts (expects 120 / 30 / 70 instead of the message_start 50 / 10 / 20), and one where the delta omits or under-reports them (expects the message_start values to stay). The first fails on main; the second guards against regressions.
• End-to-end smoke: built @posthog/ai, pointed the real @anthropic-ai/sdk through PostHogAnthropic at a local HTTP server that streams a real SSE sequence (message_start with input 50 / cache 10 / 20, final message_delta with input 120 / cache 30 / 70 and output 25), and read the captured event. On main the event reports input 50, cache creation 10, cache read 20, output 25. With this change it reports input 120, cache creation 30, cache read 70, output 25. No API keys were used.
• Checks: tests/anthropic.test.ts passes (9 passed; the remaining tests in that file are skipped without ANTHROPIC_API_KEY), oxlint reports 0 errors and oxfmt --check is clean on the touched files.
• Edge cases considered: delta omits a field, delta reports a smaller value, no web search (counts unchanged), message_start missing (previous ?? 0 behavior kept).

Release info Sub-libraries affected

Libraries affected

• @posthog/ai

Checklist

• Tests for new code
• Accounted for the impact of any changes across different platforms
• Accounted for backwards compatibility of any changes (no breaking changes!)
• Took care not to unnecessarily increase the bundle size

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

• Written with Claude Code (Claude Sonnet 5.5): the bug was found by comparing with the merged posthog-python fix, and the change, tests, smoke run and this description were produced by the agent. Agent-authored, needs human review.
• The fix keeps the larger of the message_start and message_delta values rather than overwriting, since it also tolerates a delta that omits or under-reports a field.

🤖 Generated with Claude Code

[marandaneto]

The code looks good; no qualifying findings. Verdict: correct.