Plugins Phase 5: agent-facing hooks and context injection #9

Part of the plugin system epic #3. Depends on #7 and #8. Phase 5 lets plugins shape what agents see and do inside a turn.

• Turn lifecycle hooks. (2026-10-31, feat/plugins-server-hooks: turns.onRuntimeRequest answers approvals and questions, and send hooks can replace or block a message.) Partly built in feat(plugins): run-completed hook, richer send hook and agent plugin tools #21: beforeDispatch gets the message id, attachment summaries and the requested model; turns.afterRunCompleted delivers every ended run with a stored cursor (one per registration, in order). onRuntimeRequest is not built: answering approvals needs per-plugin rules the user sets. Original text: turns.beforeDispatch (from Phase 3) gains the full message with attachments and the thread's model selection; add turns.afterRunCompleted(run) (reacts to run completion events with the run's items available for reading), turns.onRuntimeRequest(request) (approval or question raised by the provider; plugin may auto-answer under rules the user configured in its settings). Each hook is an event reaction over streamStoredEventsFrom({ afterSequence, eventType }) with the plugin's acknowledged sequence stored in PluginStorage, so a plugin that was off when the run finished catches up on start. streamDomainEvents starts at the latest sequence and cannot be used for catch-up. Shape modelled on saphid's feat(server): deliver finished runs to plugins from an acknowledged cursor pingdotgg/t3code#16048.
• Context injection. (2026-10-31: send hooks append or replace text, and web plugins add @ items, shown as chips since 2026-11-01, see Plugins Phase 4: web plugin runtime and UI contribution registries #8.) A plugin can register context.providers that contribute text blocks to the outgoing turn, each shown as a composer chip on web so the user sees what was added and can remove it for that message. Server records which plugin added what in the message metadata.
• Skills and instructions. (2026-10-31, feat/plugins-server-hooks: a skills/ folder and agents.addInstructions reach Claude and Codex. ACP agents, OpenCode, Cursor, Pi and Muse take neither from T3. 2026-11-01, feat/plugins-server-gaps: the $ picker lists plugin skills for Claude and Codex as <plugin>: <skill>.) A plugin directory may contain skills/ in the same format the composer's $ skill picker already understands (ServerProviderSkill). Plugin skills appear in the picker labelled with the plugin name, for every provider that supports skill injection.
• Slash commands with server handlers. (2026-10-31, feat/plugins-server-hooks: slashCommands.register lists the command for every client through the server-supplied list and runs the handler at dispatch, which returns text to send or a block. 2026-11-01, feat/plugins-server-gaps, with Ash's approval for one Orchestrator.ts command: a handler can return { reply }, recorded with the command as an already finished run, so no agent turn starts and every client shows it unchanged.) Delivered through the existing server-supplied provider-slash-command menu variant (see Phase 4b). A plugin slash command may rewrite the message, dispatch a different message, or answer directly without a model call (rendered through the plg_ dynamic tool renderer from Phase 4c).
• MCP tool policy. (Done in Phase 3.) Plugins can mark their tools as read or write (Phase 3) and the thread's runtime mode gates write tools the same way built-ins are gated.
• Agent-facing management. (feat(plugins): run-completed hook, richer send hook and agent plugin tools #21; both tools are writes and appear only once a plugin is installed.) MCP tools t3_plugins_invoke (call a plugin RPC by name, write access) so an agent can drive a plugin, and t3_plugins_set_enabled guarded as a write.

Measurements

message.dispatch latency with zero hooks registered: unchanged from Phase 3. With one no-op hook: report the added milliseconds.

Tests

A beforeDispatch hook that appends a context block shows up in the persisted message and in the provider input (use the Replay dev provider). afterRunCompleted fires once per run and catches up after a restart from the stored cursor. An onRuntimeRequest hook that answers an approval results in the approval event. A plugin skill appears in the skill list for Claude.

Done when

Draft PR feat(plugins): turn lifecycle hooks, context injection and plugin skills on the fork.