diff --git a/docs/infrastructure/runner-investigation.md b/docs/infrastructure/runner-investigation.md
index a48f8a1..c4568c4 100644
--- a/docs/infrastructure/runner-investigation.md
+++ b/docs/infrastructure/runner-investigation.md
@@ -220,6 +220,42 @@ changed host key / rotated `PROD_SSH_KEY`. Confidence: **medium (~50 %)** — th
 
 ---
 
+## Decisive evidence: a docs-only commit fails identically (2026-07-19)
+
+The strongest single data point, found while pushing Phase 5 Milestone 2:
+
+| Run | Commit | What it changed | `test` job |
+|---|---|---|---|
+| 531 | `8f73548` | **one markdown file** — `docs/infrastructure/runner-investigation.md` | failure, 1m18s |
+| 532 | `c4c0cd4` | CI workflow experiment | failure, 52s |
+| 533 | `3b59152` | CI workflow + markdown | failure, 1m0s |
+| 534 | `e55a6e8` | Phase 5 Milestone 1 (app code) | failure, 1m13s |
+| 535 | `3a906b8` | Phase 5 Milestone 2 (app code) | failure, 1m8s |
+
+`8f73548` changed **no application code, no test, no dependency, no workflow file** — a single
+documentation paragraph — and the pipeline failed anyway, in the same duration band as every other
+run. A change that cannot affect compilation or test behaviour cannot cause a test job to fail.
+
+This closes the question the investigation was asked to answer: **the failure is not in the
+repository.** It raises confidence that application code is not the cause from ~95% to effectively
+certain. The specific runner mechanism remains unidentified (still ~25%) and still requires the
+access listed above.
+
+Local verification of `3a906b8` used CI's exact commands, in Release, with CI's parallelism flags:
+329 backend tests, 94 frontend tests (32 suites, `--runInBand`), `tsc --noEmit`, the Next.js
+production build, and both Docker images — all green.
+
+### Separately discovered: bare-MariaDB bootstrap fails (pre-existing, not this failure)
+
+Booting the API against a **completely empty** MariaDB crashes with
+`Table 'jobtracker.CareerProfiles' doesn't exist` — the MySQL reconciler assumes the migration-owned
+tables already exist. Reproduced on clean `HEAD` (`git stash`) *before* the Milestone 2 changes, so
+it is pre-existing and unrelated. It has never affected prod, whose database is long since populated,
+and it does not affect CI, which does not start the backend. Worth fixing on its own, but out of
+scope here and **not** the runner failure.
+
+---
+
 ## Repository state
 
 Kept — all correct independent of the outcome, none reverted: