feat(providers): add Pi coding agent

[StiensWout]

Problem

Pi users depend on their existing models, auth, extensions, skills, context files, and native session history. T3 Code did not have a Pi provider, and a shallow CLI wrapper would lose the customization and session behavior that make Pi useful.

Solution

This adds Pi as an Early Access provider on Orchestrator V2:

• T3 speaks Pi's stdio JSONL RPC protocol through the user's own installation and discovers models, thinking levels, commands, skills, and auth from Pi itself.
• The thinking picker asks Pi for its effective configured level, badges that real choice as Default, and keeps inheritance internally so T3 does not override Pi.
• T3 threads use Pi's native session files for resume, current-branch fork, rollback, steering, and session naming.
• Extension dialogs, notifications, status rows, MCP tools, and resumable subagents project into the existing T3 lifecycle surfaces.
• The launcher replaces only Pi's conflicting official subagent extension, deduplicates explicit extension arguments, and restores trusted user and project extensions.
• Pi's session statistics feed the shared composer context meter. The ring tracks the active thread's context window while the popover exposes cumulative tokens processed across that Pi session.
• Turn settlement, retry and compaction recovery, child cancellation, process teardown, and fork rollback paths fail closed without leaking or corrupting the active native session.

The MCP bridge, resumable subagent work, and per-model thinking support originated in @mwolson's stacked contribution and retain that authorship.

Related work

Closes #397.
Closes #402.
Addresses #6685.

This supersedes the Pi implementations in #2211, #2748, #2800, #2812, #2831, #2856, #3818, #3947, #4355, #4445, #5688, #5882, and #6319.

This incorporates StiensWout#34 by @mwolson.

Validation

• 35 focused Pi tests, reduced from 48 by removing duplicate helper and branch matrices while strengthening observable lifecycle coverage.
• Targeted contracts, server, and web typechecks.
• Targeted changed-file lint and formatting.
• Both generated Pi extensions parsed independently as TypeScript.
• Live validation against Pi 0.84.2, including provider discovery, multi-turn context usage, MCP, extension UI, native subagents, steering, rollback, and provider handoff.
• Mobile parity audited for the shared Codex and Claude surfaces; Pi-specific mobile behavior is intentionally limited to provider identity and model selection.

Proof

Provider discovery

End-to-end Pi turn

Context meter persists across turns

The active context stays at 21k/272k while Pi's cumulative session total rises from 90k to 111k after the next completed turn.

Configured thinking default

Pi's effective setting appears as the labeled default while the internal selection continues to inherit from Pi.

Native resumable subagent

Native steering

Checkpoint and Pi session-tree rollback

Stacked on #2829 (t3code/codex-turn-mapping) until that branch lands.

Built by GPT-5.6 Sol in T3 Code through Codex.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 561e4289-385d-455d-bdde-05ea7f17a38d

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[macroscopeapp]

Effect service conventions review of the new Pi provider code. Two error-modeling issues found; everything else (namespace subpath imports, Effect.catchTags usage, layer/driver construction acquiring ChildProcessSpawner/FileSystem/IdAllocatorV2/ServerConfig from the environment, Schema.TaggedErrorClass failures) follows the conventions.

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

One consistency finding on the new web provider icon wiring. Everything else in the web scope (providerDriverMeta.ts, ProviderModelsSection.tsx, contextWindow.ts, session-logic.ts, AddProviderInstanceDialog.tsx) is registry/data-only and matches the existing per-driver patterns.

^{Posted via Macroscope — UI Consistency}

[macroscopeapp]

Approvability

Verdict: Needs human review

Diff is too large for automated approval analysis. A human reviewer should evaluate this PR.

^{You can customize Macroscope's approvability policy. Learn more.}

[macroscopeapp]

One finding on the Pi icon geometry after the tile removal. Everything else in the web scope (provider icon map, driver meta, model placeholder, display-name and picker option entries) follows the existing per-provider patterns.

^{Posted via Macroscope — UI Consistency}

[macroscopeapp]

Effect service conventions: one finding — a raw child-process stderr payload is copied into a log annotation. Elsewhere in this repo process output is reported as lengths only (stdoutLength/stderrLength, lineLength), with explicit tests asserting stderr is not retained in diagnostics.

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

Two log annotations in the new Pi adapter copy Pi's raw wire error text into the observability stream. Everything else in the Pi files (error attributes, cause handling, driver/layer shape, dependency acquisition) matches the repo's adapter conventions.

^{Posted via Macroscope — Effect Service Conventions}