[ZouR-Ma]

Problem

The Step system prompt's <env> block carried Git branch and Uncommitted changes, read from git whenever the prompt was rebuilt. The prompt is not rebuilt per user message; it is rebuilt when tools or resources change (an MCP catalog refresh or restart, an extension registering tools, /reload). After a checkout, such an unrelated rebuild re-read git and rewrote the system prompt. Anthropic-style prompt caching matches the request prefix (tools, then system, then history), so that one rewrite turned the whole conversation into cache writes. Between rebuilds the model saw a stale branch, and a run that checked out a branch mid-run (the agent itself running git checkout) never saw the change.

Reproduced with the real CLI in RPC mode against a recording Anthropic endpoint: switching branches changed nothing until an MCP server restart with an identical tool catalog, which then changed the system prompt hash while the tool hash stayed the same.

Changes

• src/step/system-prompt.ts: drop the git lines, the synchronous git reads and their 5 s cache from <env>. encodeEnvironmentValue is exported for reuse.
• src/features/step-runtime-context.ts (new), registered in step-capabilities.ts:
  • On before_agent_start, read the branch with one git symbolic-ref --quiet --short HEAD (exit 0 branch, 1 detached HEAD, 128 not a repository; anything else, such as a missing binary or timeout, announces nothing). Persist a hidden step-runtime-context custom message beside the prompt only when the branch differs from the newest snapshot in the active context. The snapshot says it supersedes earlier ones, so history stays append-only.
  • Mark the branch stale after a step that ran tools, and for runs started without before_agent_start (goal continuation, cron, background subagent wake-ups, approved plans, retries). The context hook then re-reads git once and, when the request does not already show the current branch, appends the snapshot at the request tail without writing history. The same hook restores a snapshot that compaction summarized away. The next user prompt persists the new branch.
• src/index.ts: export createStepRuntimeContextExtension for SDK embedders that use buildStepSystemPromptAppendix without the capabilities extension.
• CHANGELOG.md: entry under Unreleased → Changed.

Behavior and compatibility

• The system prompt no longer contains Git branch or Uncommitted changes, so it and the tool block stay byte-identical across checkouts and unrelated rebuilds.
• The model sees the branch as a user-role <system-reminder> snapshot instead. A detached HEAD is stated as such, and leaving a repository after a branch was announced produces a "not inside a git repository" snapshot. Outside a repository, nothing is announced.
• The uncommitted-file count is removed, not moved: it changes with almost every edit the agent makes, which would append a snapshot nearly every turn, and the prompt already instructs the model to inspect git state before changing files.
• Existing sessions are unaffected; older sessions simply have no snapshot until the next prompt.
• SDK hosts that compose only buildStepSystemPromptAppendix lose the branch line unless they add createStepRuntimeContextExtension.

Validation

Commits, each passing the pre-commit pnpm run check:

1. fix(coding-agent): keep git state out of the cached system prompt: prompt change, per-prompt snapshot, compaction restore, tests.
2. fix(coding-agent): refresh the git branch within a run: mid-run and promptless-run refresh, tests.
3. feat(coding-agent): export the runtime-context extension
4. docs(changelog): record the git runtime-context change

• CI build-check-test and Check commit attribution: passed.
• Focused tests from packages/coding-agent at commit 1 (34/34) and at the tip (40/40): pnpm exec vitest --run test/step-runtime-context.test.ts test/suite/regressions/git-runtime-context-cache-prefix.test.ts test/step-system-prompt.test.ts test/step-capabilities-extension.test.ts test/suite/regressions/plan-mode-prompt-cache-prefix.test.ts.
• An earlier CI run on this branch failed in test/json-mode-durability.test.ts with exit 143 (its 60 s spawnSync cap killed the CLI child). main run 42374504621 failed the same way in that file without this change. Locally the file passes, about 6 s per case even with every core saturated, and a network trace of the same child shows only local IPC connections, so I did not change that test here.
• test/suite/regressions/git-runtime-context-cache-prefix.test.ts drives a real session with real git:
  • a checkout between prompts plus a forced prompt rebuild keeps the system prompt and tools byte-identical, history append-only, and appends one new snapshot. It fails against the previous system-prompt.ts (not to contain 'Git branch:').
  • a tool that runs git checkout inside one run shows Git branch: dev at the tail of the next request with an unchanged prefix. It fails against commit 1's extension.
• ./test.sh (macOS):
  • With the default $TMPDIR, test/subagent-invocation.test.ts failed with listen EINVAL: tsx's IPC socket path under the isolated temp root is 113 characters, over the macOS socket path limit. The file passes (4/4) when run directly.
  • With TMPDIR=/tmp ./test.sh: scripts, config, contracts, telemetry, lark-adapter, providers (484 passed), agent-core (560 passed), coding-agent (5342 passed) and tui passed. apps/cli had 1 failure in test/edit-tool-no-full-redraw.test.ts ("keeps the applied change on a Step card whose call failed after writing"), a render-timing assertion in the edit tool card. The file passed 5/5 in three consecutive direct runs; it does not touch the system prompt or this extension.

Limitations

• Within a run, a new branch is shown at the request tail and persisted only on the next user prompt; a run started without a user prompt never persists one.
• A branch changed outside the session (another terminal) between steps that ran no tools is noticed at the next user prompt or the next step that runs tools.
• Each user prompt pays one git symbolic-ref call (bounded by a 1.5 s timeout).
• Today's date remains in <env>; crossing midnight still rewrites the system prompt on the next rebuild, at most once a day.