docs: add docs/API_REFERENCE.md (JSON envelope, error shape, output format contract)

Author: code-yeongyu

PARITY.md

@@ -213,5 +213,5 @@ Repo documentation suite shipped during extended dogfood audit. Status: present/
 | CODE_OF_CONDUCT.md | ✅ Present | Low | Contributor Covenant v2.1 |
 | .github/PULL_REQUEST_TEMPLATE.md | ✅ Present | Medium | Standardizes PR descriptions |
 | docs/CONFIGURATION.md | ✅ Present | High | env vars, settings.json, provider config — relates to #283, #285 |
-| docs/API_REFERENCE.md | ❌ Missing | Medium | JSON envelope schema, output format contract |
+| docs/API_REFERENCE.md | ✅ Present | Medium | JSON envelope schema, output format contract — #288, #266, #168c |
 | .github/ISSUE_TEMPLATE/bug_report.md | ✅ Present | #431 | Standard bug template with repro steps, environment, context sections |

docs/API_REFERENCE.md

@@ -0,0 +1,174 @@
+# API Reference — JSON Output Envelope Contract
+
+This document describes the machine-readable JSON output emitted by `claw` when
+`--output-format json` is passed.  All JSON envelopes are written to **stdout**.
+Stderr is reserved for non-contractual diagnostics only (see pinpoint #168c).
+
+---
+
+## Output Format Flag
+
+```
+claw [command] --output-format json
+claw [command] --output-format text   # default
+```
+
+When `json` is active, **all** output (success and error) is emitted as a single
+JSON object on stdout.  Consumers must not parse stderr for errors.
+
+---
+
+## Success Envelope — `claw -p <prompt>`
+
+Full non-compact run (default):
+
+```json
+{
+  "message": "<final assistant text>",
+  "model": "claude-opus-4-5",
+  "iterations": 3,
+  "auto_compaction": null,
+  "tool_uses": [...],
+  "tool_results": [...],
+  "prompt_cache_events": [...],
+  "usage": {
+    "input_tokens": 1234,
+    "output_tokens": 567,
+    "cache_creation_input_tokens": 0,
+    "cache_read_input_tokens": 0
+  },
+  "estimated_cost": "$0.0123"
+}
+```
+
+Compact run (`--compact`):
+
+```json
+{
+  "message": "<final assistant text>",
+  "compact": true,
+  "model": "claude-opus-4-5",
+  "usage": {
+    "input_tokens": 1234,
+    "output_tokens": 567,
+    "cache_creation_input_tokens": 0,
+    "cache_read_input_tokens": 0
+  }
+}
+```
+
+### Field Reference
+
+| Field | Type | Description |
+|---|---|---|
+| `message` | string | Final assistant reply text |
+| `model` | string | Model identifier used for the turn |
+| `iterations` | integer | Number of tool-use / re-prompt iterations |
+| `compact` | boolean | Present and `true` when `--compact` mode was active |
+| `auto_compaction` | object\|null | Non-null when auto-compaction fired (see below) |
+| `tool_uses` | array | Tool calls made during the turn (TODO: verify schema) |
+| `tool_results` | array | Results returned to the model (TODO: verify schema) |
+| `prompt_cache_events` | array | Cache-hit/miss events (TODO: verify schema) |
+| `usage.input_tokens` | integer | Input tokens billed |
+| `usage.output_tokens` | integer | Output tokens billed |
+| `usage.cache_creation_input_tokens` | integer | Tokens written to prompt cache |
+| `usage.cache_read_input_tokens` | integer | Tokens served from prompt cache |
+| `estimated_cost` | string | Human-readable USD cost estimate (e.g. `"$0.0123"`) |
+
+#### `auto_compaction` sub-object
+
+```json
+{
+  "removed_messages": 12,
+  "notice": "Auto-compacted: removed 12 messages to free context."
+}
+```
+
+---
+
+## Error Envelope
+
+When a command fails under `--output-format json`, an error envelope is written
+to **stdout** (pinpoint #168c / #288):
+
+```json
+{
+  "type": "error",
+  "error": "<short human-readable reason>",
+  "kind": "<snake_case error kind token>",
+  "hint": "<optional actionable hint>"
+}
+```
+
+### Error Envelope Fields
+
+| Field | Type | Description |
+|---|---|---|
+| `type` | string | Always `"error"` |
+| `error` | string | Short prose description of the failure |
+| `kind` | string | Machine-readable snake_case token (see §Error Kinds) |
+| `hint` | string\|null | Optional remediation hint |
+
+### Error Kinds (selected)
+
+`kind` values are classified by `classify_error_kind()`.  Common tokens include:
+
+- `not_yet_implemented` — command stub not yet shipped
+- `config_error` — configuration file parse / validation failure
+- `auth_error` — API key or credential problem
+- `permission_denied` — tool-use permission denied
+- `model_error` — upstream model API error
+
+See pinpoint #266 (typed-error-kind) for the full taxonomy.
+
+---
+
+## Streaming Behavior
+
+`claw` always uses streaming internally (HTTP chunked transfer to the Anthropic
+API) but the **JSON output envelope is emitted once**, after the turn completes.
+There is no per-token or per-chunk JSON stream exposed to the caller.
+
+In REPL / interactive mode (`claw` with no `-p`) the JSON format applies only to
+structured sub-commands, not to the interactive session itself.
+
+---
+
+## Status Snapshot (`claw status`)
+
+```json
+{
+  "kind": "status",
+  "status": "ok",
+  "config_load_error": null,
+  "model": "claude-opus-4-5",
+  "model_source": "config",
+  "model_raw": null,
+  "permission_mode": "default",
+  "usage": {
+    "messages": 42,
+    "turns": 10,
+    "latest_total": 5678,
+    "cumulative_input": 12345,
+    "cumulative_output": 4567,
+    "cumulative_total": 16912,
+    "estimated_tokens": 16912
+  },
+  "workspace": {
+    "cwd": "/Users/you/project",
+    "project_root": "/Users/you/project",
+    "git_branch": "main",
+    "git_state": "clean",
+    "changed_files": 0
+  }
+}
+```
+
+---
+
+## Related Pinpoints
+
+- **#288** — error-envelope stdout emission contract
+- **#266** — typed-error-kind taxonomy
+- **#168c** — `--output-format json` routes error envelopes to stdout
+- **#247** — JSON envelope field preservation (hint / help text)