diff --git a/CLAUDE.md b/CLAUDE.md
index d1d12e1..ec3b2b7 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -32,7 +32,7 @@ uv run pytest
 
 ## Architecture
 
-> Quick orientation. The authoritative, code-current account of each capability lives in [`architecture/`](architecture/).
+> Quick orientation. The authoritative, code-current account of each capability lives in [`architecture/`](architecture/). **When a change alters a capability's behavior, update the matching `architecture/<capability>.md` in the same PR** — that promotion is what keeps `architecture/` true; code that changes without it silently rots the truth home.
 
 ### Scope hierarchy
 
diff --git a/planning/changes/2026-06-25.02-architecture-promotion-reminder/change.md b/planning/changes/2026-06-25.02-architecture-promotion-reminder/change.md
new file mode 100644
index 0000000..c864070
--- /dev/null
+++ b/planning/changes/2026-06-25.02-architecture-promotion-reminder/change.md
@@ -0,0 +1,54 @@
+---
+date: 2026-06-25
+slug: architecture-promotion-reminder
+summary: Add an agent-facing note in CLAUDE.md reminding that a behavior change must promote to the matching architecture/<capability>.md in the same PR.
+outcome: Added a one-line promotion imperative to CLAUDE.md's `## Architecture` intro, resolving evaluation Finding 3 with an agent-facing reminder rather than a CI gate.
+---
+
+# Change: Agent-facing architecture/ promotion reminder
+
+**Lane:** lightweight — one file (`CLAUDE.md`), a few lines, no code, no test.
+
+## Goal
+
+Close the last unenforced step of the planning convention: the `architecture/`
+promotion. The whole convention hinges on hand-editing the affected
+`architecture/<capability>.md` in the implementing PR, and it is the easiest step
+to forget. This is evaluation **Finding 3**.
+
+We deliberately reject the heavier options (a git-diff heuristic that warns/blocks
+when `modern_di/` changes without an `architecture/` change, with an escape
+hatch). That heuristic is inherently fuzzy — most code changes (bugfixes,
+refactors, perf) legitimately touch no capability contract — so it would produce
+frequent false positives and train cargo-cult acknowledgments, exactly the
+ceremony this convention has been shedding. A reminder at the point an agent
+reads the code is enough.
+
+## Approach
+
+Add a single agent-facing imperative to `CLAUDE.md`'s `## Architecture` intro
+blockquote — the spot an agent consults to orient on a capability *before*
+changing its behavior. It names the action and the *why*. `architecture/README.md`'s
+"Promotion rule" stays the canonical statement for the truth-home audience; the
+CLAUDE.md line is a deliberate one-line echo aimed at agents (who reliably load
+`CLAUDE.md` but rarely open `architecture/README.md`).
+
+New blockquote text:
+
+> Quick orientation. The authoritative, code-current account of each capability
+> lives in [`architecture/`](architecture/). **When a change alters a
+> capability's behavior, update the matching `architecture/<capability>.md` in
+> the same PR** — that promotion is what keeps `architecture/` true; code that
+> changes without it silently rots the truth home.
+
+## Files
+
+- `CLAUDE.md` — extend the `## Architecture` intro blockquote with the promotion
+  imperative.
+
+## Verification
+
+- [ ] Apply the edit to `CLAUDE.md`.
+- [ ] `just check-planning` — `planning: OK` (this bundle's frontmatter is valid).
+- [ ] `just lint-ci` — clean.
+- [ ] `just docs-build` — succeeds (`CLAUDE.md` is outside the docs build; confirms no collateral breakage).