diff --git a/tools/observer-self-assessment-api.mjs b/tools/observer-self-assessment-api.mjs
new file mode 100644
index 00000000..ebd0291a
--- /dev/null
+++ b/tools/observer-self-assessment-api.mjs
@@ -0,0 +1,207 @@
+/**
+ * tools/observer-self-assessment-api.mjs
+ *
+ * Phase 3 deferred follow-up #5: real LLM self-assessment API call.
+ *
+ * Exports:
+ *   buildSelfAssessmentPrompt({ prompt, recommendedNode, actualNode, chainExecuted })
+ *   callSelfAssessmentApi({ prompt, recommendedNode, actualNode, chainExecuted,
+ *                           apiKey, baseUrl, model, fetchImpl, timeoutMs, abortSignal })
+ *   readRuntimeFlag(name, { homedir, fsImpl })
+ *
+ * All functions are pure / fail-quiet — they never throw in production.
+ * callSelfAssessmentApi always returns string | null (null = skip self-assessment).
+ */
+
+import { join } from 'path';
+import { existsSync, readFileSync } from 'fs';
+import { homedir as osHomedir } from 'os';
+
+// ---------------------------------------------------------------------------
+// Prompt builder (pure)
+// ---------------------------------------------------------------------------
+
+/**
+ * Build the self-assessment prompt for Sonnet.
+ *
+ * System: Russian instruction asking Claude to evaluate its own routing choice
+ * and return a JSON object with 4 fields.
+ *
+ * User: interpolates the 4 context fields.
+ *
+ * @param {object} opts
+ * @param {string|null|undefined} opts.prompt             — the user's original prompt text
+ * @param {string|null|undefined} opts.recommendedNode    — node recommended by router
+ * @param {string|null|undefined} opts.actualNode         — node actually chosen / 'direct'
+ * @param {string[]|null|undefined} opts.chainExecuted    — list of chain steps executed
+ * @returns {{ system: string, user: string }}
+ */
+export function buildSelfAssessmentPrompt({ prompt, recommendedNode, actualNode, chainExecuted } = {}) {
+  const safePrompt = prompt ?? '';
+  const safeRecommended = recommendedNode ?? 'не определён';
+  const safeActual = actualNode ?? 'direct';
+  const safeChain = Array.isArray(chainExecuted) && chainExecuted.length > 0
+    ? chainExecuted.join(' → ')
+    : '[]';
+
+  const system = [
+    'Ты — внутренний наблюдатель роутинговой системы Claude Code.',
+    'Твоя задача — честно оценить качество роутингового решения, принятого в этой сессии.',
+    'Отвечай ТОЛЬКО валидным JSON-объектом без markdown-обёрток, ровно 4 поля:',
+    '  "summary": строка — краткое описание принятого решения (до 120 символов)',
+    '  "confidence_in_choice": число от 0.0 до 1.0 — насколько оптимальным был выбор',
+    '  "what_could_be_better": строка или null — что можно было сделать иначе',
+    '  "lesson_learned": строка или null — чему учит этот эпизод для будущих сессий',
+    'Не добавляй лишних полей. Не используй markdown. Только JSON.',
+  ].join('\n');
+
+  const user = [
+    'Контекст роутингового решения:',
+    '',
+    `Запрос пользователя: ${safePrompt || '(пусто)'}`,
+    `Рекомендованный узел роутером: ${safeRecommended}`,
+    `Фактически выбранный узел: ${safeActual}`,
+    `Выполненная цепочка: ${safeChain}`,
+    '',
+    'Оцени это решение. Верни JSON с 4 полями.',
+  ].join('\n');
+
+  return { system, user };
+}
+
+// ---------------------------------------------------------------------------
+// Runtime flag reader
+// ---------------------------------------------------------------------------
+
+/**
+ * Read a runtime flag from ~/.claude/runtime/<name>.json.
+ * Returns the "value" field from the file, or 'off' on any error.
+ *
+ * @param {string} name — flag file basename without .json
+ * @param {object} opts
+ * @param {string} [opts.homedir] — override home dir (for tests)
+ * @param {{ existsSync: Function, readFileSync: Function }} [opts.fsImpl] — override fs (for tests)
+ * @returns {string}
+ */
+export function readRuntimeFlag(name, { homedir, fsImpl } = {}) {
+  const home = homedir ?? osHomedir();
+  const fs = fsImpl ?? { existsSync, readFileSync };
+
+  try {
+    const filePath = join(home, '.claude', 'runtime', `${name}.json`);
+    if (!fs.existsSync(filePath)) return 'off';
+    const raw = fs.readFileSync(filePath, 'utf-8');
+    const parsed = JSON.parse(raw);
+    if (typeof parsed.value !== 'string') return 'off';
+    return parsed.value;
+  } catch {
+    return 'off';
+  }
+}
+
+// ---------------------------------------------------------------------------
+// API caller (async, fail-quiet)
+// ---------------------------------------------------------------------------
+
+const DEFAULT_BASE_URL = 'https://api.proxyapi.ru/anthropic';
+const DEFAULT_MODEL = 'claude-sonnet-4-6';
+const DEFAULT_TIMEOUT_MS = 10000;
+const MAX_TOKENS = 512;
+
+/**
+ * Call the Anthropic /v1/messages endpoint with the self-assessment prompt.
+ * Returns the text content from the first content block, or null on any failure.
+ *
+ * Fail-quiet contract: any error (missing key, network error, non-2xx, JSON
+ * parse error, timeout) → return null. Never throws.
+ *
+ * @param {object} opts
+ * @param {string|null|undefined} opts.prompt
+ * @param {string|null|undefined} opts.recommendedNode
+ * @param {string|null|undefined} opts.actualNode
+ * @param {string[]|null|undefined} opts.chainExecuted
+ * @param {string|null|undefined} opts.apiKey         — ROUTER_LLM_KEY value
+ * @param {string} [opts.baseUrl]                     — API base URL
+ * @param {string} [opts.model]                       — model alias
+ * @param {Function} [opts.fetchImpl]                 — override fetch (for tests)
+ * @param {number} [opts.timeoutMs]                   — abort timeout in ms
+ * @param {AbortSignal} [opts.abortSignal]            — external abort signal
+ * @returns {Promise<string|null>}
+ */
+export async function callSelfAssessmentApi({
+  prompt,
+  recommendedNode,
+  actualNode,
+  chainExecuted,
+  apiKey,
+  baseUrl = DEFAULT_BASE_URL,
+  model = DEFAULT_MODEL,
+  fetchImpl,
+  timeoutMs = DEFAULT_TIMEOUT_MS,
+  abortSignal,
+} = {}) {
+  // Guard: no key → skip silently
+  if (!apiKey) return null;
+
+  const fetchFn = fetchImpl ?? globalThis.fetch;
+
+  const { system, user } = buildSelfAssessmentPrompt({ prompt, recommendedNode, actualNode, chainExecuted });
+
+  const url = `${baseUrl}/v1/messages`;
+  const body = JSON.stringify({
+    model,
+    max_tokens: MAX_TOKENS,
+    system,
+    messages: [{ role: 'user', content: user }],
+  });
+
+  // Build abort signal — wire to caller's signal if provided
+  let timeoutId;
+  let controller;
+  let signal = abortSignal;
+
+  if (!signal) {
+    controller = new AbortController();
+    signal = controller.signal;
+  }
+
+  // Build a timeout promise that resolves to null after timeoutMs.
+  // We always race the fetch against the timeout so that even when the
+  // fetchImpl ignores the AbortSignal (e.g. in tests) the timeout still wins.
+  const timeoutPromise = new Promise((resolve) => {
+    timeoutId = setTimeout(() => resolve(null), timeoutMs);
+    if (controller) {
+      // Also abort the controller so real fetch() implementations cancel early.
+      setTimeout(() => controller.abort(), timeoutMs);
+    }
+  });
+
+  try {
+    const fetchPromise = fetchFn(url, {
+      method: 'POST',
+      headers: {
+        'content-type': 'application/json',
+        'x-api-key': apiKey,
+        'authorization': `Bearer ${apiKey}`,
+        'anthropic-version': '2023-06-01',
+      },
+      body,
+      signal,
+    }).then(async (response) => {
+      if (!response.ok) return null;
+      const data = await response.json();
+      const text = data?.content?.[0]?.text;
+      if (typeof text !== 'string') return null;
+      return text;
+    }).catch(() => null);
+
+    // Race: first settlement wins.
+    const result = await Promise.race([fetchPromise, timeoutPromise]);
+    return result ?? null;
+  } catch {
+    // Unexpected outer error → fail-quiet
+    return null;
+  } finally {
+    if (timeoutId !== undefined) clearTimeout(timeoutId);
+  }
+}
diff --git a/tools/observer-self-assessment-api.test.mjs b/tools/observer-self-assessment-api.test.mjs
new file mode 100644
index 00000000..33ce996e
--- /dev/null
+++ b/tools/observer-self-assessment-api.test.mjs
@@ -0,0 +1,260 @@
+/**
+ * Tests for tools/observer-self-assessment-api.mjs
+ * Phase 3 deferred follow-up #5: real LLM self-assessment API call.
+ * TDD — these tests are written BEFORE the implementation exists.
+ */
+
+import { describe, it, expect } from 'vitest';
+import {
+  buildSelfAssessmentPrompt,
+  callSelfAssessmentApi,
+  readRuntimeFlag,
+} from './observer-self-assessment-api.mjs';
+
+// ---------------------------------------------------------------------------
+// 1. buildSelfAssessmentPrompt — all 4 fields interpolated
+// ---------------------------------------------------------------------------
+describe('buildSelfAssessmentPrompt — all fields interpolated', () => {
+  it('returns system+user strings with all 4 fields present in user string', () => {
+    const { system, user } = buildSelfAssessmentPrompt({
+      prompt: 'напиши тест для биллинга',
+      recommendedNode: '#62',
+      actualNode: '#19',
+      chainExecuted: ['#19', '#62'],
+    });
+    expect(typeof system).toBe('string');
+    expect(system.length).toBeGreaterThan(0);
+    expect(typeof user).toBe('string');
+    expect(user).toContain('напиши тест для биллинга');
+    expect(user).toContain('#62');
+    expect(user).toContain('#19');
+    expect(user).toContain('#62'); // part of chainExecuted serialisation
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 2. buildSelfAssessmentPrompt — handles missing/null inputs gracefully
+// ---------------------------------------------------------------------------
+describe('buildSelfAssessmentPrompt — null/undefined inputs', () => {
+  it('returns valid strings when all inputs are undefined/null', () => {
+    const { system, user } = buildSelfAssessmentPrompt({});
+    expect(typeof system).toBe('string');
+    expect(typeof user).toBe('string');
+    // Should contain fallback placeholders, not throw
+    expect(user).not.toContain('undefined');
+    expect(user).not.toContain('[object Object]');
+  });
+
+  it('handles null recommendedNode and empty chainExecuted', () => {
+    const { user } = buildSelfAssessmentPrompt({
+      prompt: 'test',
+      recommendedNode: null,
+      actualNode: 'direct',
+      chainExecuted: [],
+    });
+    expect(user).toContain('test');
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 3. callSelfAssessmentApi — returns null when apiKey is missing/empty
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — missing apiKey', () => {
+  it('returns null immediately when apiKey is falsy (no fetch call)', async () => {
+    let fetchCalled = false;
+    const fakeFetch = async () => { fetchCalled = true; };
+
+    const result = await callSelfAssessmentApi({
+      prompt: 'x', recommendedNode: '#1', actualNode: '#1', chainExecuted: [],
+      apiKey: '',
+      fetchImpl: fakeFetch,
+    });
+
+    expect(result).toBeNull();
+    expect(fetchCalled).toBe(false);
+  });
+
+  it('returns null when apiKey is undefined', async () => {
+    const result = await callSelfAssessmentApi({
+      prompt: 'x', recommendedNode: '#1', actualNode: '#1', chainExecuted: [],
+      apiKey: undefined,
+    });
+    expect(result).toBeNull();
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 4. callSelfAssessmentApi — returns text on 200 + content[0].text
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — successful 200 response', () => {
+  it('returns content[0].text on ok response', async () => {
+    const responseText = '{"summary":"chose correctly","confidence_in_choice":0.9,"what_could_be_better":null,"lesson_learned":null}';
+    const fakeFetch = async () => ({
+      ok: true,
+      json: async () => ({
+        content: [{ type: 'text', text: responseText }],
+      }),
+    });
+
+    const result = await callSelfAssessmentApi({
+      prompt: 'do something',
+      recommendedNode: '#19',
+      actualNode: '#19',
+      chainExecuted: ['#19'],
+      apiKey: 'test-key',
+      baseUrl: 'https://api.example.com/anthropic',
+      model: 'claude-sonnet-4-6',
+      fetchImpl: fakeFetch,
+      timeoutMs: 5000,
+    });
+
+    expect(result).toBe(responseText);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 5. callSelfAssessmentApi — returns null on non-2xx (r.ok=false)
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — non-2xx response', () => {
+  it('returns null when response.ok is false', async () => {
+    const fakeFetch = async () => ({
+      ok: false,
+      status: 429,
+      json: async () => ({ error: { message: 'rate limited' } }),
+    });
+
+    const result = await callSelfAssessmentApi({
+      prompt: 'x', recommendedNode: '#1', actualNode: '#1', chainExecuted: [],
+      apiKey: 'test-key',
+      fetchImpl: fakeFetch,
+      timeoutMs: 5000,
+    });
+
+    expect(result).toBeNull();
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 6. callSelfAssessmentApi — returns null on fetch throw
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — fetch throws', () => {
+  it('returns null (fail-quiet) when fetch throws a network error', async () => {
+    const fakeFetch = async () => { throw new Error('network error'); };
+
+    const result = await callSelfAssessmentApi({
+      prompt: 'x', recommendedNode: '#1', actualNode: '#1', chainExecuted: [],
+      apiKey: 'test-key',
+      fetchImpl: fakeFetch,
+      timeoutMs: 5000,
+    });
+
+    expect(result).toBeNull();
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 7. callSelfAssessmentApi — returns null on timeout
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — timeout', () => {
+  it('returns null when fetch never resolves within timeoutMs', async () => {
+    // fakeFetch returns a promise that never resolves
+    const fakeFetch = async (_url, _opts) => new Promise(() => { /* never */ });
+
+    const start = Date.now();
+    const result = await callSelfAssessmentApi({
+      prompt: 'x', recommendedNode: '#1', actualNode: '#1', chainExecuted: [],
+      apiKey: 'test-key',
+      fetchImpl: fakeFetch,
+      timeoutMs: 30, // 30 ms timeout — very fast for test
+    });
+    const elapsed = Date.now() - start;
+
+    expect(result).toBeNull();
+    // Should resolve around the timeout, not hang indefinitely
+    expect(elapsed).toBeLessThan(500);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 8. callSelfAssessmentApi — sends correct headers and body
+// ---------------------------------------------------------------------------
+describe('callSelfAssessmentApi — request format', () => {
+  it('sends correct headers and body shape (spy fetchImpl)', async () => {
+    let capturedUrl, capturedOpts;
+    const fakeFetch = async (url, opts) => {
+      capturedUrl = url;
+      capturedOpts = opts;
+      return {
+        ok: true,
+        json: async () => ({ content: [{ type: 'text', text: 'ok' }] }),
+      };
+    };
+
+    await callSelfAssessmentApi({
+      prompt: 'test prompt',
+      recommendedNode: '#62',
+      actualNode: '#62',
+      chainExecuted: ['#62'],
+      apiKey: 'my-secret-key',
+      baseUrl: 'https://api.proxyapi.ru/anthropic',
+      model: 'claude-sonnet-4-6',
+      fetchImpl: fakeFetch,
+      timeoutMs: 5000,
+    });
+
+    expect(capturedUrl).toContain('/v1/messages');
+    const headers = capturedOpts.headers;
+    expect(headers['authorization'] || headers['x-api-key']).toBeTruthy();
+    const body = JSON.parse(capturedOpts.body);
+    expect(body.model).toBe('claude-sonnet-4-6');
+    expect(Array.isArray(body.messages)).toBe(true);
+    expect(body.messages[0].role).toBe('user');
+    expect(body.max_tokens).toBeGreaterThan(0);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// 9. readRuntimeFlag — reads value from file; returns 'off' on missing/malformed
+// ---------------------------------------------------------------------------
+describe('readRuntimeFlag', () => {
+  it('returns the value from {"value":"on"} when file exists', () => {
+    const fakeHomedir = '/fake/home';
+    const fakeFsImpl = {
+      existsSync: (p) => p.endsWith('self-assessment-mode.json'),
+      readFileSync: (_p, _enc) => '{"value":"on"}',
+    };
+
+    const result = readRuntimeFlag('self-assessment-mode', { homedir: fakeHomedir, fsImpl: fakeFsImpl });
+    expect(result).toBe('on');
+  });
+
+  it('returns "off" when file does not exist', () => {
+    const fakeFsImpl = {
+      existsSync: () => false,
+      readFileSync: () => { throw new Error('no file'); },
+    };
+
+    const result = readRuntimeFlag('self-assessment-mode', { homedir: '/fake', fsImpl: fakeFsImpl });
+    expect(result).toBe('off');
+  });
+
+  it('returns "off" on malformed JSON', () => {
+    const fakeFsImpl = {
+      existsSync: () => true,
+      readFileSync: () => 'NOT JSON',
+    };
+
+    const result = readRuntimeFlag('self-assessment-mode', { homedir: '/fake', fsImpl: fakeFsImpl });
+    expect(result).toBe('off');
+  });
+
+  it('returns "off" when value field is missing', () => {
+    const fakeFsImpl = {
+      existsSync: () => true,
+      readFileSync: () => '{"mode":"on"}', // no "value" key
+    };
+
+    const result = readRuntimeFlag('self-assessment-mode', { homedir: '/fake', fsImpl: fakeFsImpl });
+    expect(result).toBe('off');
+  });
+});
diff --git a/tools/observer-stop-hook.mjs b/tools/observer-stop-hook.mjs
index 295fc8b5..d3389e69 100644
--- a/tools/observer-stop-hook.mjs
+++ b/tools/observer-stop-hook.mjs
@@ -19,6 +19,7 @@ import { join } from 'path';
 import { sanitize, sanitizeWithCount } from './observer-pii-filter.mjs';
 import { parseTranscript, extractLastUserPromptText } from './observer-transcript-parser.mjs';
 import { detectMethodDirected, loadKnownNodes } from './observer-routing-detector.mjs';
+import { callSelfAssessmentApi, readRuntimeFlag } from './observer-self-assessment-api.mjs';
 
 const REQUIRED_FIELDS = ['task_id', 'timestamps', 'path_type', 'outcome', 'primary_rationale'];
 const V2_FIELDS = [
@@ -294,7 +295,7 @@ function currentMonth() {
 if (process.argv[1] && process.argv[1].replace(/\\/g, '/').endsWith('/observer-stop-hook.mjs')) {
   const chunks = [];
   process.stdin.on('data', (c) => chunks.push(c));
-  process.stdin.on('end', () => {
+  process.stdin.on('end', async () => {
     let ctx = {};
     try {
       const raw = Buffer.concat(chunks).toString('utf-8');
@@ -315,6 +316,23 @@ if (process.argv[1] && process.argv[1].replace(/\\/g, '/').endsWith('/observer-s
     }
     try {
       const ep = buildEpisodeFromContext(ctx, transcriptText);
+
+      // Step 3.5: self-assessment API call (fail-quiet).
+      // Only runs when the runtime flag is 'on' and ROUTER_LLM_KEY is set.
+      const saMode = readRuntimeFlag('self-assessment-mode');
+      const saApiKey = process.env.ROUTER_LLM_KEY || null;
+      if (saMode === 'on' && saApiKey) {
+        const rat = ep.primary_rationale ?? {};
+        const apiResult = await callSelfAssessmentApi({
+          prompt: ctx.prompt || null,
+          recommendedNode: rat.recommended_node || null,
+          actualNode: rat.node_chosen || null,
+          chainExecuted: rat.chain_executed || [],
+          apiKey: saApiKey,
+        });
+        ep.self_assessment = buildSelfAssessment({ apiResult });
+      }
+
       // Always write the episode first — exit-0-safe (spec §5.1 step 1).
       appendEpisode(ep);
       // Then the routing-gate (spec §5.1 steps 2-4).