engine-internals git:20260301.68eac18A

Immutable. This exact content never changes and is served at /api/v1/blob/f0c01b0f8c9a2bf8.

---
description: Coding Agent engine implementation details (Cursor + Claude Code) — reference when modifying engine.ts or debugging Agent behavior
globs: src/engine.ts, src/index.ts, examples/e2e-test.ts, examples/e2e-headless.ts, examples/e2e-claude-code.ts
alwaysApply: false
---

# Cursor Engine Implementation Details

## Agent CLI Invocation

- Binary path: `~/.local/bin/agent` (not `cursor`, nor `cursor agent`)
- Uses standard `child_process.spawn` — no PTY (node-pty) needed since CLI version 2026.02+
- Key flags: `--output-format stream-json --stream-partial-output --force --trust --sandbox disabled --approve-mcps`

## stream-json Output Format

Output is clean NDJSON on stdout; `stripAnsi()` is retained as a safety net but is not expected to be needed.

One JSON object per line, main types:

```
{ "type": "system", "subtype": "init", "session_id": "xxx", ... }     → Init; does not signal end of conversation
{ "type": "assistant", "message": { "content": [{ "type": "text", "text": "..." }] } }  → Assistant text delta
{ "type": "tool_call", "subtype": "started", "tool_call": { "<XxxToolCall>": { "args": {...} } } }  → Tool call started
{ "type": "tool_call", "subtype": "completed", "tool_call": { "<XxxToolCall>": { "args": {...}, "result": {...} } } }  → Tool call completed
{ "type": "result", "subtype": "success", "is_error": false, "session_id": "xxx", "duration_ms": 1234 }  → Normal end
{ "type": "result", "subtype": "error", "is_error": true, "result": "error msg" }  → Error end
```

### Key Notes

1. **tool_call has both started and completed events** — started → yield `tool_call`, completed → yield `tool_result`; do not treat both as tool_call (would duplicate)
2. **tool_call name is not a fixed field** — iterate over the `tool_call` object keys and find the one ending with `ToolCall`; some tools use `function` format (`{ "name": "...", "arguments": "..." }`)
3. **`--stream-partial-output` changes assistant event granularity** — with it, each assistant event is character-level delta; without it, full paragraphs. **Critical**: after all deltas for each segment (text between tool calls), Cursor sends one more summary event (content = concatenation of that segment’s deltas). CursorEngine uses accumulated text comparison to detect and skip summary, avoiding duplicate text
4. **assistant.message.content is an array** — multiple blocks; filter those with `type === 'text'` and concatenate
5. **system event is not a termination signal** — do not end the stream on system event
6. **session_id appears in both system and result** — use the one in result as the final session_id

## apiKey Authentication Passthrough (Cursor)

Headless environments (cloud servers, CI/CD) have no `agent login` state; must authenticate via API Key. Full passthrough chain:

```
CreateAssistantOpts.apiKey (user-provided)
  → index.ts: stored in closure, passed to engine.invoke() on each doChat()
    → InvokeOpts.apiKey
      → CursorEngine:
        1. args.push('--api-key', opts.apiKey)   // CLI argument
        2. env.CURSOR_API_KEY = opts.apiKey       // env var (backup)
      → ClaudeCodeEngine:
        1. env.ANTHROPIC_API_KEY = opts.apiKey    // env var
```

CLI layer (`cli.ts`) `run` and `serve` commands both support `--api-key <key>` (generic, any engine).

When apiKey is not passed, the engine inherits `process.env` (including existing `CURSOR_API_KEY` or `ANTHROPIC_API_KEY`); works if the host process already sets those env vars.

## durationMs Extraction

Cursor result event includes `duration_ms` (integer, milliseconds). `parseStreamLine` extracts it and puts it in the `done` event’s `durationMs` field.

- Has `duration_ms` → `{ type: 'done', sessionId, durationMs: 4500 }`
- No `duration_ms` → `{ type: 'done', sessionId, durationMs: undefined }`
- `duration_ms: 0` → `{ type: 'done', sessionId, durationMs: 0 }` (valid value)
- error result → do not expose durationMs (return error event)

## Session Resume

- Flag: `--resume <sessionId>`
- On resume failure, Agent process may exit with error (exit code != 0) or return is_error in result event
- GolemBot fallback: on resume-related error → clearSession → retry once without --resume
- Resume failure keywords: error message contains "resume" or "session" (case-insensitive)

## Skill Injection

- Cursor discovers Skills from `.cursor/skills/` directory
- GolemBot strategy: symlink `skills/<name>` to `.cursor/skills/<name>`
- Before each invoke, remove old symlinks (only those that are isSymbolicLink), then recreate
- Ensures Skill list always matches `skills/` directory

## Skill Auto-Discovery and Usage (Important)

After reading AGENTS.md and SKILL.md from .cursor/skills/, the Coding Agent **decides on its own** when to use which Skill.
- Test prompts should not explicitly say "use xxx script" or "follow xxx Skill"
- Correct approach: describe the task in natural language and let the Agent decide
- Assertions should be looser (Agent may use different filenames or output directly in the reply without writing a file)

## stdout Buffer Handling

- stdout `data` events may fire at arbitrary byte boundaries (not by line)
- Must maintain a buffer, split on `\n`, keep the last incomplete line
- On process close, drain the remaining buffer (append `\n` then process)
- After close, check whether done/error was already yielded; if not, emit an error

# Claude Code Engine Implementation Details

## CLI Invocation

- Binary path: `~/.local/bin/claude`
- Uses standard `child_process.spawn` (**no PTY needed**); reads stream-json from stdout
- Key flags: `-p <prompt> --output-format stream-json --verbose --dangerously-skip-permissions`
- `--verbose` is **required**; without it, stream-json only outputs the final result event, not intermediate assistant/user events

## stream-json Output Format

No ANSI escape sequences (plain stdout JSON); no stripAnsi needed.

One JSON object per line, main types:

```
{ "type": "system", "subtype": "init", "session_id": "xxx", ... }  → Ignore
{ "type": "assistant", "message": { "content": [{ "type": "text", "text": "..." }, { "type": "tool_use", "name": "Bash", "input": {...} }] } }  → May yield multiple StreamEvents
{ "type": "user", "message": { "content": [{ "type": "tool_result", "content": "..." }] } }  → Tool execution result
{ "type": "result", "is_error": false, "session_id": "xxx", "duration_ms": 1234, "total_cost_usd": 0.01, "num_turns": 2 }  → done
```

### Key Notes

1. **Single message can have mixed content blocks** — assistant message content array can include both text and tool_use. `parseClaudeStreamLine()` returns `StreamEvent[]` not a single event
2. **`--verbose` is required** — without it, only result is output; no intermediate stream events at all
3. **tool_result content has two formats** — string or `[{ type: "text", text: "..." }]` array; parser must handle both
4. **result event extended fields** — `total_cost_usd`, `num_turns`, `duration_api_ms`, `usage`; mapped to `StreamEvent.done`’s `costUsd` and `numTurns`
5. **No segmentAccum dedup needed** — Claude Code does not have summary events like Cursor

## Skill Injection

- Claude Code natively discovers Skills from `.claude/skills/` directory (mirrors Cursor’s `.cursor/skills/`)
- `injectClaudeSkills()` also generates `CLAUDE.md` (extra context)
- Skill frontmatter (name, description, etc.) is natively recognized by Claude Code

## Authentication

- `claude auth login` for interactive login
- `ANTHROPIC_API_KEY` env var (headless)
- apiKey passthrough: `InvokeOpts.apiKey → env.ANTHROPIC_API_KEY`