2.1.9 Test (2.1.9)

Claude Code 2.1.9 is the "hooks 2.0" release, shipping ${CLAUDE_SESSION_ID} substitution, PreToolUse additionalContext injection, and a plansDirectory setting -- collectively the most significant capability expansion since the hooks/skills foundation in 2.1.0.

"hooks become middleware, not just gates."

Epistemic status: changelog-derived and partially runtime-verified (2026-01-16). Three features were runtime-tested; two were queued as manual-only; one received code-review status without full runtime confirmation.

What shipped

${CLAUDE_SESSION_ID} substitution replaces the literal token with the session UUID at skill load time. Any skill .md file can embed the variable; the harness resolves it before the model reads the prompt. This enables session-scoped file paths such as /tmp/state-${CLAUDE_SESSION_ID}.json -- a low-ceremony working memory: write on one skill invocation, read on the next, auto-scoped to the session lifetime.

PreToolUse additionalContext lets hook commands return a JSON object with an additionalContext key. The harness surfaces that string to the model as a system reminder before the tool executes. Before 2.1.9, hooks could only approve, deny, or modify tool inputs; they could not communicate back to the model. A security hook can now annotate a sensitive file access; a compliance hook can surface a policy reminder inline.

plansDirectory is a settings key (e.g., {"plansDirectory": "./plans"}) that relocates plan-mode artifacts to a configurable path. Placing plans inside the repo makes them versionable and reviewable artifacts rather than ephemeral scratch files.

What the test found

Runtime tests on 2026-01-16 confirmed two core features. ${CLAUDE_SESSION_ID} passed: the UUID appeared correctly in skill text and was preserved across fork boundaries. additionalContext passed: context returned by a PreToolUse hook appeared to the model as a system reminder. The plansDirectory setting received CODE REVIEW status -- the path mechanism was inspected in source but not fully exercised at runtime. Three remaining features (auto:N MCP threshold, Ctrl+G in AskUserQuestion, session URL in commits) required interactive or MCP infrastructure and were deferred to manual testing queues.

One noted gap: ${CLAUDE_SESSION_ID} is available only inside skill markdown, not inside hook command strings. A hook cannot directly reference a session-scoped file using the variable; the skill must write the session ID to a known location for the hook to retrieve it, which limits the combined session-aware injection pattern.

Why it matters

The combination ${CLAUDE_SESSION_ID} + additionalContext creates a working-memory loop: a skill writes session state, a hook reads it, and the hook injects it as model context before each tool call. This pattern -- skill writes state, hook reads state, hook injects state as context -- was not expressible before 2.1.9. The plansDirectory setting is understated relative to its team utility: architectural decisions become versioned, reviewable repo artifacts instead of ephemeral plan buffers.