diff --git a/CHANGELOG.md b/CHANGELOG.md
index dc464949..d4e6c3b6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -43,6 +43,7 @@
 
 ### Added
 
+- Four failures a client had to tell apart by their sentence have a stable `code` (#1000, API contract 1.79.0, additive). The full async build queue answers 429 `build_queue_full` — it shared its status with `auth_throttled` and had no code. An authentication failure answers `unauthorized`, or `token_expired` when the bearer token's `exp` has passed, or 503 `auth_unavailable` when the JWKS could not be fetched. The overload 503 written to the socket before a request is read answers `server_overloaded`. Every body keeps its `error` text. A run a restart interrupted got its code, `credentials_required`, with #996.
 - `card.json` is declared in the contract as `DatasetCard` and states the licence mismatch and "no processing" as fields (#955, API contract 1.75.0, additive). The card sections every Gold output has carried since 1.71.0 (#694) had no schema, and two facts lived only in sentences a client had to match: `…; the provider declares: …` and `No transformation declared: values are as the source gave them.` Each `provenance` entry now also has `license_declared` (the BuildSpec's licence as written, with its link; null when none), `license_provider` (the kpubdata catalog's terms; null when none) and `license_mismatch` (both known and the provider's terms are not in the declared licence — the condition the sentence already used), and the card has `processing_declared` (false when only the no-transformation sentence is listed; a composed output's join counts as processing). The sentences are unchanged for the README. `GET /artifacts/{run_id}/{file_path}` says `gold/<source>/card.json` is a `DatasetCard` and gains an `application/json` example of one, so `contract/fixtures/responses.json` carries card fixtures. Tests validate real `card_sections` output, from the functions and from single and composed builds, against the schema, and fail when the writer sends a field the schema does not name. Cards written before 1.75.0 lack the four fields.
 - `GET /admin/runs` gives `total`, how many runs there are before `limit` (#948, API contract 1.73.0, additive). `count` was always the runs in the response — `len(runs)` after the `limit` cut — and is now documented as that, not a total; `total` is the build index's runs plus the queued and running jobs it does not hold yet, each counted once, with one `COUNT(*)` and a key lookup rather than reading every row (`BuildIndex.count_builds`, SQLite and CUBRID). `count < total` means older runs were left out, so an admin screen can say "N of M". A client talking to an older Builder finds `total` absent.
 - Saved analyses record the SQL they are written in (#875, step 14 of ADR 0021, API contract 1.76.0): `sql_dialect`, `engine`, `engine_version`, `query_contract_version` and `migration_required`. An existing analysis store gains the columns when Builder starts, and its analyses — saved for the Polars engine — are `legacy-polars`. Those are never re-run on DuckDB as if nothing had changed: `POST /analyses/{analysis_id}/run` answers 409 `analysis_migration_required` (Studio: kpubdata-lab/kpubdata-studio#565) until the SQL is reviewed and saved as a new analysis; reading and deleting them (which releases the snapshot hold) work as before.
diff --git a/contract/builder-api.yaml b/contract/builder-api.yaml
index e86181b0..79d6bda4 100644
--- a/contract/builder-api.yaml
+++ b/contract/builder-api.yaml
@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: KPubData Builder Service API
-  version: "1.78.0"
+  version: "1.79.0"
   description: >-
     KPubData Builder(패키지 `kpubdata-builder`) 서비스 계약. BuildSpec 검증, preview, build 실행, artifact 조회,
     계약 버전 조회를 제공한다. CLI와 HTTP service mode(#36)가 동일한 도메인 계약을
@@ -20,6 +20,7 @@ info:
     추가한다(#504, additive). v1.8.0은 stable principal별 encrypted Provider
     credential CRUD와 Provider status/test API를 추가한다(#492, additive). v1.9.0은
     `/query` 응답에 child startup과 Polars engine 실행 시간을 추가한다(#523, additive).
+    v1.79.0 gives four failures a stable `code` (#1000, additive): the full async build queue's 429 is `build_queue_full` (it was told from 429 `auth_throttled` only by its sentence), an authentication failure is `unauthorized`, `token_expired` (the bearer token's `exp` has passed — get a new token) or, with 503, `auth_unavailable` (the JWKS could not be fetched), and the overload 503 written before a request is read is `server_overloaded`. Each body keeps its `error` text.
     v1.78.0 lets the owner read a run a restart interrupted (#996, additive): `GET /builds/{run_id}` answers such a run as `failed` with the new optional `BuildJob.code` `credentials_required` and the reason in `error`, and `GET /builds/{run_id}/events` shows its `run_failed` event, where both answered 404 before; another user still gets what a missing run gets. The submitter of an async run is now recorded with its events, so this holds for runs submitted from this version on.
     v1.77.0 stops a snapshot profile's range from disclosing a single record's value (#903, additive): `ColumnRange.min`/`max` are read after removing the `range_trim` lowest and the `range_trim` highest values, the range's `status` is `trimmed` instead of `exact`, and it carries `trimmed_count`. `SnapshotProfile` gains `range_trim`, and `min_range_values` is at least `2 * range_trim + 1`. A client that showed only `exact` ranges shows none until it reads `trimmed`.
     v1.76.0 records which SQL a saved analysis is written in (#875, additive): `SavedAnalysis` gains `sql_dialect` (`duckdb`, or `legacy-polars` for one saved before the DuckDB cutover), `engine`, `engine_version`, `query_contract_version` and `migration_required`. A legacy analysis is never re-run on DuckDB: `POST /analyses/{analysis_id}/run` answers 409 `analysis_migration_required` until its SQL is reviewed and saved as a new analysis. Existing stores gain the columns on start; their rows are the legacy ones.
@@ -1454,11 +1455,17 @@ paths:
               schema:
                 $ref: "#/components/schemas/Error"
         "429":
-          description: async build queue가 가득 참
+          description: async build queue가 가득 참 (`build_queue_full`, #1000)
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/Error"
+              examples:
+                BuildQueueFull:
+                  summary: The async build queue is full
+                  value:
+                    error: async build queue is full
+                    code: build_queue_full
         "500":
           description: job 접수 실패(event 기록/큐잉 불가 — job은 생성되지 않음)
           content:
@@ -4184,7 +4191,10 @@ components:
                 code: signup_rejected
 
     Unauthorized:
-      description: X-API-Key가 누락되었거나 서버에 설정된 키와 일치하지 않음
+      description: >-
+        X-API-Key가 누락되었거나 서버에 설정된 키와 일치하지 않음, 또는 Bearer 토큰이
+        검증되지 않음. `code` 는 `unauthorized`, 토큰의 `exp` 가 지났으면
+        `token_expired` 다(#1000).
       content:
         application/json:
           schema:
@@ -4193,7 +4203,8 @@ components:
             MissingOrInvalidApiKey:
               summary: 인증 정보가 없거나 일치하지 않음
               value:
-                error: unauthorized
+                error: invalid api key
+                code: unauthorized
 
     PiiDeclarationUnavailable:
       description: >-
@@ -4408,6 +4419,13 @@ components:
             (`SignupNotApprovedError`, `components.responses.SignupNotApproved`).
             `revision_conflict`(#820, #947, 409) — a revision save or revert based on a
             stale revision; the body carries `current_revision` (`RevisionConflictError`).
+            `build_queue_full`(#1000, 429) — `POST /builds` when the async build queue
+            is full; try again later. `unauthorized` / `token_expired`(#1000, 401) — any
+            operation: the credentials were refused, or the bearer token's `exp` has
+            passed and a new token is needed. `auth_unavailable`(#1000, 503) — the
+            JWKS could not be fetched, so the credentials were not judged.
+            `server_overloaded`(#1000, 503) — too many requests in flight; the response
+            carries `Retry-After`.
         redistribution:
           description: >-
             `code: redistribution_forbidden`일 때만 온다(#688): 판정과 source별 이유.
diff --git a/contract/fixtures/responses.json b/contract/fixtures/responses.json
index a15cec92..2ff29d91 100644
--- a/contract/fixtures/responses.json
+++ b/contract/fixtures/responses.json
@@ -1,6 +1,6 @@
 {
   "fixture_format": 1,
-  "contract_version": "1.78.0",
+  "contract_version": "1.79.0",
   "probe_field": "future_optional_field",
   "rules": "A response may gain optional fields in any minor version; a client ignores fields it does not know. A required field keeps its name and type until the next major version; a client rejects a body whose required field is missing or mistyped.",
   "fixtures": [
@@ -5097,6 +5097,30 @@
       },
       "broken_path": "$.error"
     },
+    {
+      "operation_id": "submitBuild",
+      "method": "POST",
+      "path": "/builds",
+      "status": 429,
+      "example": "BuildQueueFull",
+      "current": {
+        "error": "async build queue is full",
+        "code": "build_queue_full"
+      },
+      "with_additive_fields": {
+        "error": "async build queue is full",
+        "code": "build_queue_full",
+        "future_optional_field": "added by a later minor contract version"
+      },
+      "additive_paths": [
+        "$"
+      ],
+      "required_type_broken": {
+        "error": 12345,
+        "code": "build_queue_full"
+      },
+      "broken_path": "$.error"
+    },
     {
       "operation_id": "listBuilds",
       "method": "GET",
@@ -5694,17 +5718,20 @@
       "status": 401,
       "example": "MissingOrInvalidApiKey",
       "current": {
-        "error": "unauthorized"
+        "error": "invalid api key",
+        "code": "unauthorized"
       },
       "with_additive_fields": {
-        "error": "unauthorized",
+        "error": "invalid api key",
+        "code": "unauthorized",
         "future_optional_field": "added by a later minor contract version"
       },
       "additive_paths": [
         "$"
       ],
       "required_type_broken": {
-        "error": 12345
+        "error": 12345,
+        "code": "unauthorized"
       },
       "broken_path": "$.error"
     },
diff --git a/src/kpubdata_builder/service/app.py b/src/kpubdata_builder/service/app.py
index 62a821ac..c0b21356 100644
--- a/src/kpubdata_builder/service/app.py
+++ b/src/kpubdata_builder/service/app.py
@@ -399,7 +399,10 @@ def _enforce_ownership() -> bool:
 #   is not disclosed (#903, additive).
 # 1.77.0 -> 1.78.0: a run a restart interrupted is readable by its owner — BuildJob
 #   gains the optional `code` (`credentials_required`) (#996, additive).
-API_CONTRACT_VERSION = "1.78.0"
+# 1.78.0 -> 1.79.0: stable codes for the full build queue (429 build_queue_full),
+#   authentication failures (unauthorized, token_expired, auth_unavailable) and the
+#   overload 503 (server_overloaded) (#1000, additive).
+API_CONTRACT_VERSION = "1.79.0"
 
 
 #: manifest status vocabulary (ok/failed/cancelled) → publish status vocabulary
@@ -1455,7 +1458,9 @@ def _dispatch_impl(
         # fault).
         if principal.status_code == 401:
             service._auth_throttle.record_failure(client_id)
-        return ServiceResponse(principal.status_code, {"error": principal.reason})
+        return ServiceResponse(
+            principal.status_code, {"error": principal.reason, "code": principal.code}
+        )
 
     # Successful authentication clears failure record — normal client that received
     # a few 401s due to token expiry doesn't get throttled during subsequent normal
diff --git a/src/kpubdata_builder/service/auth.py b/src/kpubdata_builder/service/auth.py
index 0a205b62..83e6014d 100644
--- a/src/kpubdata_builder/service/auth.py
+++ b/src/kpubdata_builder/service/auth.py
@@ -172,6 +172,24 @@ class AuthError:
 
     reason: str
     status_code: int = 401
+    # Set where the failure is made, so ``code`` does not depend on how ``reason`` is
+    # worded.
+    expired: bool = False
+
+    @property
+    def code(self) -> str:
+        """A stable code for the failure, so a client does not branch on ``reason`` (#1000).
+
+        ``token_expired`` — the bearer token's ``exp`` has passed: get a new token and
+        send the request again. ``auth_unavailable`` — the JWKS could not be fetched
+        (503): the credentials were not judged, try again. ``unauthorized`` — every
+        other refusal: a missing or wrong API key, a token that does not verify.
+        """
+        if self.status_code == 503:
+            return "auth_unavailable"
+        if self.expired:
+            return "token_expired"
+        return "unauthorized"
 
 
 def _is_dev_mode() -> bool:
@@ -466,7 +484,10 @@ def _verify_bearer_token(token: str) -> Principal | AuthError:
             options={"require": ["exp", "iat", "iss", "sub"]},
         )
     except jwt.PyJWTError as exc:
-        return AuthError(reason=f"invalid token: {type(exc).__name__}")
+        return AuthError(
+            reason=f"invalid token: {type(exc).__name__}",
+            expired=isinstance(exc, jwt.ExpiredSignatureError),
+        )
 
     if not payload.get("email_verified", False):
         return AuthError(reason="email not verified")
diff --git a/src/kpubdata_builder/service/build_runs_api.py b/src/kpubdata_builder/service/build_runs_api.py
index b89a8205..47189b0a 100644
--- a/src/kpubdata_builder/service/build_runs_api.py
+++ b/src/kpubdata_builder/service/build_runs_api.py
@@ -478,7 +478,11 @@ def _record_enqueue_failure() -> None:
                     raise RuntimeError("existing async build is missing snapshot")
                 return ServiceResponse(200, result.snapshot.to_body())
             case "queue_full":
-                return ServiceResponse(429, {"error": "async build queue is full"})
+                # A code of its own (#1000): the other 429 on this route's way in is
+                # `auth_throttled`, and the sentence was the only way to tell them apart.
+                return ServiceResponse(
+                    429, {"error": "async build queue is full", "code": "build_queue_full"}
+                )
             case unreachable:
                 assert_never(unreachable)
 
diff --git a/src/kpubdata_builder/service/http.py b/src/kpubdata_builder/service/http.py
index ec08273a..fe28168a 100644
--- a/src/kpubdata_builder/service/http.py
+++ b/src/kpubdata_builder/service/http.py
@@ -79,7 +79,7 @@ def _overloaded_response(allowed_origins: frozenset[str] = frozenset()) -> bytes
       (a bearer header is not one). The body is a constant and says nothing a page from
       another origin could not learn by being refused.
     """
-    body = b'{"error": "server overloaded"}'
+    body = b'{"error": "server overloaded", "code": "server_overloaded"}'
     cors = b""
     # This is written to the socket as bytes, so a configured origin with a line break
     # or a non-ASCII character would corrupt the response: send no CORS header then.
diff --git a/tests/unit/test_oidc_auth.py b/tests/unit/test_oidc_auth.py
index d1a7b787..b0def735 100644
--- a/tests/unit/test_oidc_auth.py
+++ b/tests/unit/test_oidc_auth.py
@@ -8,6 +8,7 @@
 
 import logging
 import time
+from pathlib import Path
 
 import jwt
 import pytest
@@ -271,6 +272,8 @@ def test_expired_token(self, oidc_env: bytes) -> None:
         token = _make_token(oidc_env, exp=now - 120)
         result = authenticate(bearer_token=f"Bearer {token}")
         assert isinstance(result, AuthError)
+        # A code of its own (#1000): the client gets a new token instead of reading the reason.
+        assert result.code == "token_expired"
 
     def test_wrong_audience(self, oidc_env: bytes) -> None:
         token = _make_token(oidc_env, aud="wrong-client")
@@ -770,6 +773,57 @@ def test_non_owner_denied_via_legacy_path(self) -> None:
         assert not principal_owns(created_by="oidc:a", owner_id=None, principal=principal)
 
 
+class TestStableAuthCodes:
+    """An authentication failure says what kind it is without its sentence (#1000)."""
+
+    def test_a_refused_credential_is_unauthorized(self, oidc_env: bytes) -> None:
+        result = authenticate(bearer_token=f"Bearer {_make_token(oidc_env, aud='wrong-client')}")
+
+        assert isinstance(result, AuthError)
+        assert (result.status_code, result.code) == (401, "unauthorized")
+
+    def test_the_code_does_not_read_the_sentence(self) -> None:
+        # ``token_expired`` is set where the failure is made; rewording ``reason`` — or a
+        # reason that happens to end the same way — does not change the code.
+        assert AuthError(reason="the token is past its exp", expired=True).code == "token_expired"
+        assert AuthError(reason="invalid token: ExpiredSignatureError").code == "unauthorized"
+
+    @pytest.mark.parametrize("sent", [None, "wrong-key"])
+    def test_a_missing_key_and_a_wrong_key_get_the_contract_example(
+        self, sent: str | None, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # The contract's one ``Unauthorized`` example stands for both, so both must
+        # answer with its sentence.
+        monkeypatch.setenv("KPUBDATA_BUILDER_API_KEY", "secret-key")
+
+        result = authenticate(api_key=sent)
+
+        assert isinstance(result, AuthError)
+        assert (result.reason, result.code) == ("invalid api key", "unauthorized")
+
+    def test_an_unreachable_jwks_is_auth_unavailable(self) -> None:
+        assert AuthError(reason="auth service unavailable (jwks)", status_code=503).code == (
+            "auth_unavailable"
+        )
+
+    def test_the_response_body_carries_the_code(
+        self, oidc_env: bytes, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        from kpubdata_builder.service import BuilderService, ServiceResponse, dispatch
+
+        service = BuilderService(output_root=tmp_path, client_factory=lambda **_kw: None)
+        expired = _make_token(oidc_env, exp=int(time.time()) - 120)
+
+        response = dispatch(service, "GET", "/datasets", None, bearer_token=f"Bearer {expired}")
+
+        assert isinstance(response, ServiceResponse)
+        assert response.status_code == 401
+        assert response.body == {
+            "error": "invalid token: ExpiredSignatureError",
+            "code": "token_expired",
+        }
+
+
 class TestKeycloakAccessToken:
     """What Studio sends is a Keycloak access token (kpubdata-studio#722).
 
diff --git a/tests/unit/test_service.py b/tests/unit/test_service.py
index 7af29120..ce8cd2f1 100644
--- a/tests/unit/test_service.py
+++ b/tests/unit/test_service.py
@@ -1716,7 +1716,7 @@ def test_overloaded_response_is_a_well_formed_http_message(self) -> None:
         head, _, body = _OVERLOADED_RESPONSE.partition(b"\r\n\r\n")
         headers = dict(line.split(b": ", 1) for line in head.split(b"\r\n")[1:])
         assert int(headers[b"Content-Length"]) == len(body)
-        assert json.loads(body) == {"error": "server overloaded"}
+        assert json.loads(body) == {"error": "server overloaded", "code": "server_overloaded"}
 
     @staticmethod
     def _overloaded_headers(allowed: frozenset[str]) -> dict[bytes, bytes]:
@@ -1725,7 +1725,7 @@ def _overloaded_headers(allowed: frozenset[str]) -> dict[bytes, bytes]:
         head, _, body = _overloaded_response(allowed).partition(b"\r\n\r\n")
         headers = dict(line.split(b": ", 1) for line in head.split(b"\r\n")[1:])
         assert int(headers[b"Content-Length"]) == len(body)
-        assert json.loads(body) == {"error": "server overloaded"}
+        assert json.loads(body) == {"error": "server overloaded", "code": "server_overloaded"}
         return headers
 
     def test_overloaded_response_has_no_cors_header_without_an_allowed_origin(self) -> None:
diff --git a/tests/unit/test_stable_error_codes.py b/tests/unit/test_stable_error_codes.py
new file mode 100644
index 00000000..5da20a11
--- /dev/null
+++ b/tests/unit/test_stable_error_codes.py
@@ -0,0 +1,36 @@
+"""Failures a client had to tell apart by their sentence carry a stable code (#1000)."""
+
+from __future__ import annotations
+
+import json
+import threading
+from pathlib import Path
+
+from kpubdata_builder.service.http import _overloaded_response
+from tests.unit.test_service_jobs import VALID_SPEC_YAML, _BlockingBuildService
+
+
+def test_a_full_build_queue_says_so_with_a_code(tmp_path: Path) -> None:
+    """429 was shared with ``auth_throttled`` and had no code of its own."""
+    entered = threading.Event()
+    release = threading.Event()
+    service = _BlockingBuildService(
+        output_root=tmp_path, entered=entered, release=release, async_max_queue_size=1
+    )
+    try:
+        service.submit_build(VALID_SPEC_YAML, run_id="run1", created_by="tester")
+        assert entered.wait(timeout=5)
+        service.submit_build(VALID_SPEC_YAML, run_id="run2", created_by="tester")
+
+        saturated = service.submit_build(VALID_SPEC_YAML, run_id="run3", created_by="tester")
+    finally:
+        release.set()
+
+    assert saturated.status_code == 429
+    assert saturated.body == {"error": "async build queue is full", "code": "build_queue_full"}
+
+
+def test_the_overload_response_carries_its_code() -> None:
+    _head, _, body = _overloaded_response().partition(b"\r\n\r\n")
+
+    assert json.loads(body)["code"] == "server_overloaded"