diff --git a/lib/providers/anthropic.mjs b/lib/providers/anthropic.mjs
index 45a6d8d..a249990 100644
--- a/lib/providers/anthropic.mjs
+++ b/lib/providers/anthropic.mjs
@@ -967,6 +967,19 @@ async function* _spawnAndStream(irRequest, authContext, spawnImpl, isolationCtx)
     stdio: ['pipe', 'pipe', 'pipe'],
   });
 
+  // Guard stdin writes against EPIPE (child may close stdin before we finish
+  // writing — e.g. on auth error, bad model, or oversized prompt).
+  // Port of OCP server.mjs:780.
+  // Authority: claude CLI v2.1.104 observed behaviour — fast-failing spawn closes
+  // stdin before OLP finishes writing; Node stdin Writable surfaces EPIPE.
+  // ALIGNMENT.md Rule 2: no CLI operation changed; guard is a Node stream safety net.
+  // The typeof guard protects test-injected mock stdin objects that are plain
+  // objects (not EventEmitters) and do not implement .on().
+  if (typeof proc.stdin.on === 'function') {
+    proc.stdin.on('error', (e) => {
+      console.error(JSON.stringify({ ts: new Date().toISOString(), level: 'warn', event: 'stdin_write_error', error: e.message }));
+    });
+  }
   // OCP server.mjs:586-587: proc.stdin.write(prompt); proc.stdin.end();
   proc.stdin.write(prompt);
   proc.stdin.end();
diff --git a/lib/providers/codex.mjs b/lib/providers/codex.mjs
index d005387..c3aad10 100644
--- a/lib/providers/codex.mjs
+++ b/lib/providers/codex.mjs
@@ -505,6 +505,19 @@ async function* _spawnAndStream(irRequest, authContext, spawnImpl, isolationCtx)
 
   const proc = spawnImpl(finalBin, finalArgs, { env: finalEnv, stdio: ['pipe', 'pipe', 'pipe'] });
 
+  // Guard stdin writes against EPIPE (child may close stdin before we finish
+  // writing — e.g. on auth error, bad model, or oversized prompt).
+  // Port of OCP server.mjs:780.
+  // Authority: codex CLI Codex CLI reference (https://developers.openai.com/codex/cli/reference)
+  // observed behaviour — fast-failing spawn closes stdin before OLP finishes writing.
+  // ALIGNMENT.md Rule 2: no CLI operation changed; guard is a Node stream safety net.
+  // The typeof guard protects test-injected mock stdin objects that are plain
+  // objects (not EventEmitters) and do not implement .on().
+  if (typeof proc.stdin.on === 'function') {
+    proc.stdin.on('error', (e) => {
+      console.error(JSON.stringify({ ts: new Date().toISOString(), level: 'warn', event: 'stdin_write_error', error: e.message }));
+    });
+  }
   // Write prompt via stdin for multi-line prompts (D6 assumption A1)
   if (useStdin) {
     proc.stdin.write(prompt);
diff --git a/lib/providers/mistral.mjs b/lib/providers/mistral.mjs
index 8a4c7b1..7c29844 100644
--- a/lib/providers/mistral.mjs
+++ b/lib/providers/mistral.mjs
@@ -500,6 +500,20 @@ async function* _spawnAndStream(irRequest, authContext, spawnImpl) {
 
   const proc = spawnImpl(bin, args, { env, stdio: ['pipe', 'pipe', 'pipe'] });
 
+  // Guard stdin writes against EPIPE (child may close stdin before we finish
+  // writing — vibe --prompt reads from CLI args, but stdin.end() can still
+  // surface EPIPE if the process exits before the fd is flushed).
+  // Port of OCP server.mjs:780.
+  // Authority: Mistral Vibe CLI https://docs.mistral.ai/mistral-vibe/terminal/quickstart
+  // observed behaviour — fast-failing spawn closes stdin; Node Writable surfaces EPIPE.
+  // ALIGNMENT.md Rule 2: no CLI operation changed; guard is a Node stream safety net.
+  // The typeof guard protects test-injected mock stdin objects that are plain
+  // objects (not EventEmitters) and do not implement .on().
+  if (typeof proc.stdin.on === 'function') {
+    proc.stdin.on('error', (e) => {
+      console.error(JSON.stringify({ ts: new Date().toISOString(), level: 'warn', event: 'stdin_write_error', error: e.message }));
+    });
+  }
   // Close stdin immediately — vibe --prompt reads from args, not stdin.
   proc.stdin.end();