diff --git a/CHANGELOG.md b/CHANGELOG.md
index 13e138593..902a2c498 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **E2E coverage for the retired-route diagnosis** (`tests/e2e/test_retired_labs_route_diagnosis_e2e.py`,
+  `e2e_auth`, 0 credits, one read-only GET). The unit tests for
+  [#875](https://github.com/ffroliva/gflow-cli/issues/875) feed the classifier a *captured*
+  body, so they stay green if Flow changes the wording, un-retires the route or switches to
+  401 — while the user-facing diagnosis silently goes wrong again. This one asks Flow. It
+  needs no project fixture (the route is retired before any project lookup) and **skips
+  rather than passes** on an account still served the labs tRPC API, so a cohort change
+  cannot retire it unnoticed. Verified by falsification: with detection disabled it fails
+  with the pre-fix message verbatim.
+
 ### Fixed
 
 - **A retired labs route no longer tells you to simplify a prompt you never wrote.**
diff --git a/docs/E2E_TESTING.md b/docs/E2E_TESTING.md
index e06e206be..ceebcb160 100644
--- a/docs/E2E_TESTING.md
+++ b/docs/E2E_TESTING.md
@@ -402,9 +402,35 @@ tests/
     ├── test_image_batch_e2e.py           # [e2e, e2e_batch]
     ├── test_video_t2v_e2e.py             # [e2e, e2e_video]
     ├── test_incident_quality_e2e.py      # [e2e, e2e_auth] incident-bundle quality benchmark — 0 credits
+    ├── test_retired_labs_route_diagnosis_e2e.py  # [e2e, e2e_auth] #875 — 0 credits, no project fixture
     └── test_data_layer_e2e.py            # [e2e, e2e_{image,video,data}]
 ```
 
+### Diagnosing a surface Flow has retired
+
+`test_retired_labs_route_diagnosis_e2e.py` (marker `e2e_auth`, **0 credits**,
+one read-only GET) guards what gflow *says* when a labs tRPC route answers
+`404 "Flow RPCs have been deprecated and disabled"` — that it names the retired
+route instead of telling the user to check a payload that is fine, simplify a
+prompt that does not exist, or file a bug for a condition Flow documents in the
+body being classified (#875).
+
+It is the rare case where the offline test genuinely cannot substitute. The unit
+tests in `tests/api/test_client_errors.py` feed the classifier a *captured* body;
+they stay green if Flow changes the wording, un-retires the route, or switches to
+401, while the user-facing diagnosis silently goes wrong again.
+
+Two deliberate choices make it runnable on any profile in the nightly canary:
+
+- **No project fixture.** The route is retired before any project lookup, so the
+  diagnosis is project-independent and a syntactically valid id suffices.
+- **It skips rather than passes** when the account is still served the labs tRPC
+  API, because a silent green would let a cohort change retire the test without
+  anyone noticing — the `skip_on_migrated_host` failure mode in reverse.
+
+Verified by falsification on 2026-09-20: with detection disabled it fails with
+the pre-fix message verbatim, so it is a guard rather than a test that can only pass.
+
 ### Incident-bundle quality benchmark
 
 `test_incident_quality_e2e.py` (marker `e2e_auth`, **0 credits**) is not a
diff --git a/tests/e2e/test_retired_labs_route_diagnosis_e2e.py b/tests/e2e/test_retired_labs_route_diagnosis_e2e.py
new file mode 100644
index 000000000..f08ba52aa
--- /dev/null
+++ b/tests/e2e/test_retired_labs_route_diagnosis_e2e.py
@@ -0,0 +1,91 @@
+"""Live proof that a retired labs tRPC route diagnoses itself honestly (#875).
+
+Opt-in: ``-m e2e_auth`` with ``GFLOW_CLI_E2E_PROFILE`` set. One read-only GET.
+**$0** — no generation, no credits, no writes to the account.
+
+Why this test exists as an e2e and not only as a unit test: the unit tests in
+``tests/api/test_client_errors.py`` feed ``_raise_for_non_retryable`` a captured
+body and assert the remediation. That proves our classifier does what we think.
+It cannot prove Flow still answers the way we captured it — which is the whole
+premise of this repo. If Flow un-retires the route, changes the wording, or
+starts answering 401 instead, the unit tests stay green and the user-facing
+diagnosis silently goes wrong again. Only a live call can tell.
+
+No project fixture on purpose. The route is retired *before* any project lookup
+happens, so the diagnosis is project-independent and a syntactically valid id is
+enough — which is what lets this run on any profile in the nightly canary with
+no extra configuration.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from gflow_cli.api.client import FlowApiClient
+from gflow_cli.errors import WireFormatError
+
+pytestmark = [pytest.mark.e2e, pytest.mark.e2e_auth]
+
+#: Syntactically valid, deliberately not a real project. The retired route never
+#: gets as far as resolving it (measured 2026-09-20: the 404 is identical for a
+#: project the account owns).
+_ANY_PROJECT = "00000000-0000-4000-8000-000000000000"
+
+#: What v0.79.0 and earlier told the user, verbatim from ``WireFormatError``'s
+#: class default. Each phrase is wrong on this response in its own way.
+_PAYLOAD_CLAIM = "Check request payload parameters"
+_PROMPT_CLAIM = "simpler prompt"
+_BUG_CLAIM = "File a bug"
+
+
+async def test_a_retired_labs_route_does_not_blame_the_payload_or_a_prompt(
+    e2e_profile_dir: Path,
+) -> None:
+    """``character list`` is a READ. It has no prompt and no payload to simplify.
+
+    Two outcomes are correct and this asserts the invariant across both, which is
+    what lets it run on a cohort we do not control and cannot choose:
+
+    * the route answers and characters come back (an account still served the
+      labs tRPC API), or
+    * the route is retired, and the failure says so **without** telling the user
+      to check a payload that is fine, simplify a prompt that does not exist, or
+      file a bug for a condition Flow documents in the very body we classified.
+    """
+    async with FlowApiClient(profile_dir=e2e_profile_dir) as client:
+        try:
+            characters = await client.list_characters(_ANY_PROJECT)
+        except WireFormatError as exc:
+            if "deprecated" not in (exc.detail or "").lower():
+                # A different wire failure. Not this test's subject, and asserting
+                # on it would make an unrelated outage look like a #875 regression.
+                pytest.skip(f"not the retirement refusal: {exc.detail!r}")
+
+            hint = exc.remediation_hint or ""
+            assert _PAYLOAD_CLAIM not in hint, f"blames a payload that is fine: {hint}"
+            assert _PROMPT_CLAIM not in hint, (
+                f"asks to simplify a prompt that does not exist: {hint}"
+            )
+            assert _BUG_CLAIM not in hint, f"asks for a bug report Flow already answered: {hint}"
+
+            # It must still say something actionable rather than merely stop.
+            assert "projectInitialData" in hint, f"does not name the retired route: {hint}"
+            assert "639" in hint, f"does not point at the port that would fix it: {hint}"
+
+            # MEASURED 2026-09-20 on denon82, both on `auto` and with the host pinned:
+            # GFLOW_CLI_FLOW_HOST=flow.google.com returns the SAME 404, because this
+            # read has no migrated arm. Suggesting it would repeat the exact failure
+            # #875 fixed, one field over.
+            assert "GFLOW_CLI_FLOW_HOST" not in hint, (
+                f"suggests a setting measured not to help: {hint}"
+            )
+        else:
+            # A served route is a correct outcome, but it exercised none of #875.
+            # Skip rather than pass: the canary runs -m e2e_auth nightly, and a
+            # silent green here would let a cohort change retire this test without
+            # anyone noticing — the failure mode `skip_on_migrated_host` was written
+            # for, in reverse.
+            assert isinstance(characters, list)
+            pytest.skip("labs tRPC still served for this account — #875 branch not exercised")
diff --git a/website/docs/E2E_TESTING.md b/website/docs/E2E_TESTING.md
index e06e206be..ceebcb160 100644
--- a/website/docs/E2E_TESTING.md
+++ b/website/docs/E2E_TESTING.md
@@ -402,9 +402,35 @@ tests/
     ├── test_image_batch_e2e.py           # [e2e, e2e_batch]
     ├── test_video_t2v_e2e.py             # [e2e, e2e_video]
     ├── test_incident_quality_e2e.py      # [e2e, e2e_auth] incident-bundle quality benchmark — 0 credits
+    ├── test_retired_labs_route_diagnosis_e2e.py  # [e2e, e2e_auth] #875 — 0 credits, no project fixture
     └── test_data_layer_e2e.py            # [e2e, e2e_{image,video,data}]
 ```
 
+### Diagnosing a surface Flow has retired
+
+`test_retired_labs_route_diagnosis_e2e.py` (marker `e2e_auth`, **0 credits**,
+one read-only GET) guards what gflow *says* when a labs tRPC route answers
+`404 "Flow RPCs have been deprecated and disabled"` — that it names the retired
+route instead of telling the user to check a payload that is fine, simplify a
+prompt that does not exist, or file a bug for a condition Flow documents in the
+body being classified (#875).
+
+It is the rare case where the offline test genuinely cannot substitute. The unit
+tests in `tests/api/test_client_errors.py` feed the classifier a *captured* body;
+they stay green if Flow changes the wording, un-retires the route, or switches to
+401, while the user-facing diagnosis silently goes wrong again.
+
+Two deliberate choices make it runnable on any profile in the nightly canary:
+
+- **No project fixture.** The route is retired before any project lookup, so the
+  diagnosis is project-independent and a syntactically valid id suffices.
+- **It skips rather than passes** when the account is still served the labs tRPC
+  API, because a silent green would let a cohort change retire the test without
+  anyone noticing — the `skip_on_migrated_host` failure mode in reverse.
+
+Verified by falsification on 2026-09-20: with detection disabled it fails with
+the pre-fix message verbatim, so it is a guard rather than a test that can only pass.
+
 ### Incident-bundle quality benchmark
 
 `test_incident_quality_e2e.py` (marker `e2e_auth`, **0 credits**) is not a