diff --git a/.changeset/claude-edit-read-guard.md b/.changeset/claude-edit-read-guard.md
new file mode 100644
index 0000000..d10d4ab
--- /dev/null
+++ b/.changeset/claude-edit-read-guard.md
@@ -0,0 +1,5 @@
+---
+"@colony/hooks": patch
+---
+
+Remind Claude Code to read existing files before edit tools so Update/Edit/MultiEdit calls do not stop with "File must be read first".
diff --git a/CLAUDE.md b/CLAUDE.md
index 89f7b0c..e1ebfa0 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -20,7 +20,8 @@ The signature property of the project is that **memory is stored compressed**. E
 8. **Local by default.** Default embedding provider is local (Transformers.js). Remote providers are opt-in via settings. Do not add default network calls.
 9. **No silent failures.** Hook and worker errors are logged as structured JSON; user-visible commands surface failures with a non-zero exit code and a short message.
 10. **No daemon on the write path.** Hooks write observations synchronously through `MemoryStore.addObservation` — never across a network or HTTP boundary. Hooks may *detach-spawn* the worker to kick off background embedding, but they must never wait on it. If the worker is down, writes still succeed; only the semantic-search side is degraded (BM25 keeps working).
-11. **Never edit on the local base branch.** Treat the local `main` checkout as read-only. Every task — even a typo or one-line fix — runs on a dedicated `agent/*` branch inside a worktree. Do not run `git checkout main` / `git switch main` to start work, do not `git commit` on the primary working tree, and do not push to `main` directly. This matches what codex does via Guardex and keeps parallel lanes safe.
+11. **Read before edit tools.** Claude Code rejects `Edit` / `Update` / `MultiEdit` on an existing file unless that exact file path was read first in the current session. Before any edit tool call, run `Read` on the target file with the same relative path you will edit.
+12. **Never edit on the local base branch.** Treat the local `main` checkout as read-only. Every task — even a typo or one-line fix — runs on a dedicated `agent/*` branch inside a worktree. Do not run `git checkout main` / `git switch main` to start work, do not `git commit` on the primary working tree, and do not push to `main` directly. This matches what codex does via Guardex and keeps parallel lanes safe.
 
 ## Worktree discipline
 
diff --git a/packages/hooks/src/handlers/user-prompt-submit.ts b/packages/hooks/src/handlers/user-prompt-submit.ts
index ef1d8f1..720980a 100644
--- a/packages/hooks/src/handlers/user-prompt-submit.ts
+++ b/packages/hooks/src/handlers/user-prompt-submit.ts
@@ -7,6 +7,8 @@ import type { HookInput } from '../types.js';
  * so one chatty agent can't drown out the signal in another agent's context.
  */
 const MAX_INJECTED_MESSAGES = 6;
+const CLAUDE_EDIT_READ_GUARD =
+  'Claude Code edit safety: Read each existing target file before Edit/Update/MultiEdit, or Claude Code rejects with "File must be read first".';
 
 export async function userPromptSubmit(store: MemoryStore, input: HookInput): Promise<string> {
   // Compute the "since" cursor *before* recording this turn's prompt so the
@@ -24,7 +26,8 @@ export async function userPromptSubmit(store: MemoryStore, input: HookInput): Pr
   const activity = buildTaskUpdatesPreface(store, input.session_id, sinceTs);
   const conflicts = buildConflictPreface(store, input.session_id);
   const pheromones = buildPheromoneConflictPreface(store, input.session_id);
-  return [activity, conflicts, pheromones].filter(Boolean).join('\n\n');
+  const editGuard = input.ide === 'claude-code' ? CLAUDE_EDIT_READ_GUARD : '';
+  return [editGuard, activity, conflicts, pheromones].filter(Boolean).join('\n\n');
 }
 
 /**
diff --git a/packages/hooks/test/runner.test.ts b/packages/hooks/test/runner.test.ts
index 1d04596..bc9471f 100644
--- a/packages/hooks/test/runner.test.ts
+++ b/packages/hooks/test/runner.test.ts
@@ -52,6 +52,33 @@ describe('runHook', () => {
     expect(tl[0]?.content).not.toMatch(/basically/i);
   });
 
+  it('user-prompt-submit reminds Claude Code to read files before edit tools', async () => {
+    const claude = await runHook(
+      'user-prompt-submit',
+      {
+        session_id: 'sess-claude-edit',
+        ide: 'claude-code',
+        prompt: 'Update packages/hooks/src/handlers/user-prompt-submit.ts',
+      },
+      { store },
+    );
+    expect(claude.ok).toBe(true);
+    expect(claude.context).toContain('Read each existing target file before Edit/Update/MultiEdit');
+    expect(claude.context).toContain('File must be read first');
+
+    const codex = await runHook(
+      'user-prompt-submit',
+      {
+        session_id: 'sess-codex-edit',
+        ide: 'codex',
+        prompt: 'Update packages/hooks/src/handlers/user-prompt-submit.ts',
+      },
+      { store },
+    );
+    expect(codex.ok).toBe(true);
+    expect(codex.context ?? '').not.toContain('Edit/Update/MultiEdit');
+  });
+
   it('post-tool-use records a tool_use observation with metadata', async () => {
     await runHook('session-start', { session_id: 'sess-c', ide: 'claude-code' }, { store });
     const r = await runHook(