diff --git a/.claude/rules/typescript.md b/.claude/rules/typescript.md
index daacc44e..4a3185a6 100644
--- a/.claude/rules/typescript.md
+++ b/.claude/rules/typescript.md
@@ -24,3 +24,50 @@
 - Use Vitest `describe`/`it` blocks
 - Test file naming: `<module>.test.ts`
 - Integration tests should test via HTTP API, not internal functions
+
+## Design Patterns
+
+### Internal-mutable / external-readonly Map (issue #4779)
+
+When a module owns a Map whose mutation has non-obvious invariants (e.g. a `.finally(() => delete)` cleanup that only works if external callers never insert entries), expose a **read-only view** to consumers and keep the **mutable map** as a private field. Enforce the producer/consumer split at compile time.
+
+Shape:
+
+```ts
+// Producer (owning module — AcpBackend)
+private readonly pendingHandshakesInternal = new Map<string, PendingHandshake>();
+private readonly pendingHandshakes: ReadonlyMap<string, PendingHandshake> =
+  this.pendingHandshakesInternal;
+
+// ...mutations only via `pendingHandshakesInternal`:
+this.pendingHandshakesInternal.set(session.id, { session, backendRunId, ready });
+.finally(() => { this.pendingHandshakesInternal.delete(session.id); });
+
+// Consumer (PromptDeps and beyond)
+export interface PromptDeps {
+  // ...
+  pendingHandshakes: ReadonlyMap<string, PendingHandshake>;
+}
+```
+
+**Anchoring the boundary in the acp backend (post-#4779):**
+
+- **Producer:** `launchBackgroundHandshake` in `src/services/acp/backend.ts` — the only `.set` / `.delete` callsite, and the only caller of `pendingHandshakesInternal`. Keep the producer's mutations behind a single function so the surface stays auditable.
+- **Consumer:** `PromptDeps.pendingHandshakes` in `src/services/acp/backend/prompts.ts` — read-only access via `.get` / `.has`. New consumers see the readonly view, not the mutable field, by construction (the mutable field is `private`).
+
+Why two fields, not a single `ReadonlyMap` assignment? A single `readonly` field declared `ReadonlyMap` still has a mutable identity — TypeScript only narrows the declared type. The producer needs a real `Map` reference to call `.set` / `.delete`; the external view is a structural alias of the same object. Splitting into two fields (mutable internal + readonly public) makes the producer the only path that compiles the mutator calls, and the type system prevents external code from re-introducing them.
+
+**Test the contract, not the runtime.** Use Vitest's `expectTypeOf` to assert the public type lacks the mutator methods:
+
+```ts
+expectTypeOf<PromptDeps['pendingHandshakes']>().not.toHaveProperty('set');
+expectTypeOf<PromptDeps['pendingHandshakes']>().not.toHaveProperty('delete');
+expectTypeOf<PromptDeps['pendingHandshakes']>().not.toHaveProperty('clear');
+// Read-only methods must still exist:
+expectTypeOf<PromptDeps['pendingHandshakes']>().toHaveProperty('get');
+expectTypeOf<PromptDeps['pendingHandshakes']>().toHaveProperty('has');
+```
+
+If a future refactor widens `pendingHandshakes` back to `Map<…>`, the type-level assertions fail at `tsc` time — not at code-review time, not at runtime. The regression test makes the contract explicit.
+
+Canonical example: `src/services/acp/backend/types.ts` (`PendingHandshake` named type, post-#4779) and `src/__tests__/acp-pendinghandshakes-readonly-4779.test.ts` (the `expectTypeOf` regression test).