diff --git a/api/app/canon_log.py b/api/app/canon_log.py
index bf7e30c..d10de58 100644
--- a/api/app/canon_log.py
+++ b/api/app/canon_log.py
@@ -38,7 +38,9 @@ def _validator(schema_name: str) -> Draft202012Validator:
 
 def validation_errors(doc: dict, schema_name: str) -> list[str]:
     validator = _validator(schema_name)
-    return [e.message for e in sorted(validator.iter_errors(doc), key=lambda e: list(e.path))]
+    # Stringify path segments before sorting: a path can mix str property names
+    # and int array indices, which are not orderable against each other.
+    return [e.message for e in sorted(validator.iter_errors(doc), key=lambda e: [str(p) for p in e.path])]
 
 
 def validate_origin(doc: dict) -> list[str]:
diff --git a/api/app/main.py b/api/app/main.py
index 6ae1412..d04fa7b 100644
--- a/api/app/main.py
+++ b/api/app/main.py
@@ -7,6 +7,8 @@ Prompt routing, model selection, and logging land later.
 """
 
 from fastapi import Depends, FastAPI, HTTPException
+from fastapi.exceptions import RequestValidationError
+from fastapi.responses import JSONResponse
 from pydantic import BaseModel
 
 from .canon_log import validate_canon_log
@@ -14,6 +16,16 @@ from .canon_log import validate_canon_log
 app = FastAPI(title="coc-rpg proxy", version="0.0.1")
 
 
+@app.exception_handler(RequestValidationError)
+async def unify_validation_errors(request, exc: RequestValidationError) -> JSONResponse:
+    """Reshape pydantic body-validation failures into the same envelope as a
+    canon-log schema failure, so the client parses ONE 422 shape (charter §11):
+    {"detail": {"canon_log_errors": [...]}}.
+    """
+    errors = [f"{'.'.join(str(loc) for loc in e['loc'])}: {e['msg']}" for e in exc.errors()]
+    return JSONResponse(status_code=422, content={"detail": {"canon_log_errors": errors}})
+
+
 class TurnRequest(BaseModel):
     canon_log: dict
 
diff --git a/api/pytest.ini b/api/pytest.ini
index c7b23ec..c038626 100644
--- a/api/pytest.ini
+++ b/api/pytest.ini
@@ -1,3 +1,5 @@
 [pytest]
 pythonpath = .
 testpaths = tests
+filterwarnings =
+    ignore:Using `httpx` with `starlette.testclient` is deprecated
diff --git a/api/tests/test_endpoints.py b/api/tests/test_endpoints.py
index ee536f2..b7db5ee 100644
--- a/api/tests/test_endpoints.py
+++ b/api/tests/test_endpoints.py
@@ -36,3 +36,12 @@ def test_every_role_endpoint_rejects_an_invalid_canon_log():
 def test_missing_canon_log_is_a_422():
     r = client.post("/dm/narrate", json={})
     assert r.status_code == 422
+    assert "canon_log_errors" in r.json()["detail"]
+
+
+def test_malformed_body_uses_the_unified_error_shape():
+    # A pydantic body failure returns the SAME envelope as a schema failure,
+    # so the client only ever parses one 422 shape.
+    r = client.post("/dm/narrate", json={"canon_log": "not-a-dict"})
+    assert r.status_code == 422
+    assert "canon_log_errors" in r.json()["detail"]
diff --git a/docs/canon-log.md b/docs/canon-log.md
new file mode 100644
index 0000000..b090df6
--- /dev/null
+++ b/docs/canon-log.md
@@ -0,0 +1,110 @@
+# Canon Log — living contract
+
+The canon log is the compact structured state code maintains and injects into
+every AI call (charter §11). This is the **contract** the Godot client and the
+FastAPI proxy both bind to. The schemas are the single source of truth; this doc
+is the human-readable companion.
+
+- Schemas (authoritative): [`/docs/schemas/canon-log.schema.json`](schemas/canon-log.schema.json), [`/docs/schemas/origin.schema.json`](schemas/origin.schema.json)
+- Design rationale: [`superpowers/specs/2026-07-09-canon-log-schema-design.md`](superpowers/specs/2026-07-09-canon-log-schema-design.md)
+
+## The three layers
+
+| Layer | Owner | Where | Mutable? |
+|---|---|---|---|
+| World content | authored | `/content/world` (id-referenced) | no — static per playthrough |
+| Origin seed | authored | `/content/origins` | no — an initial-state overlay |
+| **Canon log** | **code (client)** | runtime + save | **yes — evolves every turn** |
+
+The client **constructs** the canon log at new-game (origin + world + character
+creation) and **maintains** it turn to turn. The proxy never mutates it — it
+**validates** every posted log and rejects an invalid one before any prompt work.
+
+## Canon log shape
+
+Full field rules live in `canon-log.schema.json`. Summary:
+
+```json
+{
+  "schema_version": 1,
+  "player":   { "name": "…", "class_id": "sellsword|assassin|priest", "luck_descriptor": "…" },
+  "location": { "id": "…", "name": "…" },
+  "party":    [ { "id": "…", "name": "…", "disposition": -100..100 } ],
+  "recent_events":     ["… (≤5, rolling)"],
+  "established_facts": ["… (durable, uncapped)"],
+  "active_quests":     [ { "id": "…", "name": "…", "status": "active|complete|failed", "objective": "…" } ],
+  "humiliations":      [ { "id": "…", "text": "…", "weight": 1..10, "turn": 0.. } ]
+}
+```
+
+**The §7 boundary (enforced structurally).** The schema sets
+`additionalProperties: false` on the root and on `player`, and `player` admits
+only `name`, `class_id`, `luck_descriptor`. **Numeric Luck, stats, HP/MP, and
+inventory cannot be represented in the canon log** — the AI never sees a number
+it could use to calculate Luck. Only `luck_descriptor` (e.g. "Fortune spits on
+you") crosses the boundary.
+
+All string ids match `^[a-z0-9_]+$`.
+
+## Origin seed shape
+
+Full rules in `origin.schema.json`. An origin seeds the initial log:
+
+```json
+{
+  "schema_version": 1,
+  "id": "…", "display_name": "…", "description": "…",
+  "start_location_id": "…",
+  "situation": ["…"], "opening_facts": ["…"],
+  "disposition_overrides": { "<npc_id>": -100..100 },
+  "inventory_grants": [ { "item_id": "…", "qty": 1.. } ],
+  "start_quest_id": "…|null",
+  "build_constraints": { "allowed_classes": ["sellsword",…], "luck_modifier": 0 }
+}
+```
+
+Every id an origin references must resolve in world content — the proxy exposes
+`content.unresolved_refs(origin, world)` for that check; `humiliations` are never
+seeded (earned in play).
+
+## API contract
+
+The client posts game state to a role endpoint; the body is:
+
+```json
+{ "canon_log": { …a canon log… } }
+```
+
+Endpoints (charter §4): `POST /dm/narrate`, `/dm/adjudicate`, `/dm/improvise`,
+`/npc/speak`, `/party/banter`. Each validates `canon_log` and, on success,
+returns the role's result. (Prompt routing is stubbed for now — a valid log
+returns `{"detail": "not implemented"}`.)
+
+### Error shape — ONE envelope for every 422
+
+Whether the log fails the JSON Schema **or** the request body is malformed at the
+pydantic layer (missing/mistyped `canon_log`), the proxy returns the **same**
+shape, so the client parses one thing:
+
+```json
+HTTP 422
+{ "detail": { "canon_log_errors": ["<message>", "<message>", …] } }
+```
+
+Client rule: on 422, read `detail.canon_log_errors` (a list of strings). It is
+never the raw pydantic error list. A degraded-DM fallback (charter §13) should
+fire on any non-200 rather than surfacing these strings to the player — they are
+for logs and development.
+
+## Per-role injection
+
+The canon log is the shared base of every call; roles add their own extras (the
+log never grows per-role fields):
+
+| Role | Log + … |
+|---|---|
+| Narrator | log only |
+| NPC | log + persona, `knowledge[]`, `available_moves[]` (§6) |
+| Improviser | log + the whitelisted event-change set (§7) |
+| Banter | log, with `humiliations` as primary source (§9) |
+| Adjudicator | log + the current legal action set |