feat(adapter): add amp adapter (selective passthrough)

[rynfar]

Summary

• Adds an adapter for Amp, Sourcegraph's coding agent (npm: @sourcegraph/amp).
• Selective passthrough architecture: intercept /api/provider/anthropic/* (Claude inference via Claude Max), forward every other Amp endpoint to AMP_UPSTREAM_URL (default https://ampcode.com). Threads sync, attachments, telemetry, login, usage, code review, skills, MCP, permissions all keep working — entire Amp app remains usable.
• New ampForwarder module handles HTTP and SSE streaming with proper Content-Encoding handling so Bun-fetch's auto-decoded responses don't trigger client-side ZlibError.
• Detection by path prefix (/api/provider/anthropic/) or any x-amp-* header.
• Snake_case tool mapping (create_file, edit_file, bash) for file-change extraction.
• Confirmed end-to-end against real amp CLI (v0.0.1778130306-g2889d9): inference round-trips through Claude Max, free-tier user bypasses Sourcegraph's 402 paywall on amp -x.

Setup

npm install -g @sourcegraph/amp
amp login                                    # one time, against real ampcode.com
export AMP_URL=http://127.0.0.1:3456         # then point at Meridian
export AMP_API_KEY=$(python3 -c "import json,os; print(json.load(open(f'{os.path.expanduser(\"~\")}/.local/share/amp/secrets.json'))['apiKey@https://ampcode.com/'])")
amp -x "say hi"

(AMP_API_KEY is needed because Amp keys credentials by URL — the stored key is for https://ampcode.com, not the local URL.)

Configuration

Env var	Default	Purpose
AMP_UPSTREAM_URL	https://ampcode.com	Forward target for non-inference traffic
MERIDIAN_AMP_FORWARD_DISABLED	unset	Disable forwarding entirely

Test plan

• Unit tests: src/__tests__/amp-adapter.test.ts (33 tests), src/__tests__/amp-forwarder.test.ts (10 tests)
• Detection tests in src/__tests__/adapter-detection.test.ts (7 new cases)
• HTTP-layer integration tests in src/__tests__/amp-integration.test.ts (9 tests)
• npm test — 1791 passing, 0 failing
• npm run build — clean
• Live verification matrix against real amp CLI on port 3458:
  • Inference: I1 round-trip ✓ I2 glob/grep ✓ I3 read_file ✓ I4 create_file (file written) ✓ I5 thinking blocks (with reasoning) ✓ I7 count_tokens ✓ I8 deep mode (402 from ampcode.com proves cross-provider forwarding works) ✓
  • Forwarder: T1 threads list ✓ (after Content-Encoding fix) T2 new thread ✓ T4 search ✓ S1 skill list ✓ Tl1 tools list ✓ M1 mcp list ✓ P1 permissions list ✓ A2 usage display ✓
  • Multimodal (I9) deferred for manual UI testing — not blocking.

Notable bug found and fixed during verification

amp threads list initially failed with ZlibError. Bun's native fetch() transparently decodes gzipped response bodies but preserves the original Content-Encoding: gzip header. Forwarding that header verbatim made Amp's client try to gunzip an already-decoded body. Fix: strip Content-Encoding and Content-Length from forwarded responses (commit 510c28a). Regression test included.

Spec & plan

• Design: docs/superpowers/specs/2026-05-06-amp-adapter-design.md
• Plan: docs/superpowers/plans/2026-05-06-amp-adapter.md