feat(agent): emit per-category context token breakdown#2352
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This was referenced May 25, 2026
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This was referenced May 25, 2026
Adds a `breakdown` field to the `_posthog/usage_update` ext-notification so the renderer can render per-source token splits in the upcoming context-breakdown popover. The agent estimates token counts for the stable pieces of the context (system prompt today; tools / MCP / rules / skills / subagents will follow as the agent gets at-rest access to their definitions) via a character-ratio heuristic. The `conversation` bucket is derived as `max(0, currentInputTokens - sum(stable))`, so the categories always sum to the input total. The renderer's `useContextUsage` now scans backwards for both the existing `session/update` aggregate and the new ext-notification's breakdown, surfacing the latter as a nullable field. Existing callers keep the aggregate; B4 wires the breakdown into the popover. Generated-By: PostHog Code Task-Id: bac06178-1ab1-4000-9a56-1901215bd4af Generated-By: PostHog Code Task-Id: bac06178-1ab1-4000-9a56-1901215bd4af
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Problem
The context usage indicator in the renderer showed only aggregate token counts (used / size / percentage) with no breakdown of where those tokens were coming from. Users had no way to see how much of the context window was consumed by the system prompt, tools, rules, conversation history, etc.
Changes
Agent-side (
packages/agent):context-breakdown.tswith a lightweight character-ratio token estimator (~3.5 chars/token) and helpers:estimateTokens/estimateJsonTokensfor cheap client-side estimationestimateSystemPrompthandles raw strings,{ type: "preset", append }objects, andundefined, adding a constantCLAUDE_PRESET_ESTIMATE_TOKENS(4000) when the opaque Claude preset is in usebuildBreakdownderives theconversationbucket as whatever input tokens remain after subtracting the stable pieces (system prompt, tools, rules, skills, MCP, subagents), floored at 0 to absorb estimation driftemptyBaseline/ContextBreakdownBaselinetypes for initializing and carrying per-source estimates across turnscontextBreakdownBaselineto theSessiontype, initialized at session start with the estimated system-prompt token count_posthog/usage_updatenotification after each model turn, using the result's own input token categories rather than the streamed delta to handle subagent turns correctlyRenderer-side (
apps/code):ContextUsagewith abreakdown: ContextBreakdown | nullfield and added theContextBreakdowninterface (mirroring the agent shape, kept local to avoid a cross-package dependency)extractContextUsageto scan events in a single reverse pass, independently finding the latest aggregate (session/update) and the latest breakdown (_posthog/usage_updateor__posthog/usage_update), then merging themHow did you test this?
context-breakdown.tscoveringestimateTokens,estimateJsonTokens,estimateSystemPrompt, andbuildBreakdown(edge cases: empty input, circular JSON, conversation floored at 0, preset vs. raw string)extractContextUsagecovering: no events, aggregate-only, merged breakdown, and the double-underscore method prefix variantPublish to changelog?
no