diff --git a/app/api/search.py b/app/api/search.py
index d77b6a1..0ed58f1 100644
--- a/app/api/search.py
+++ b/app/api/search.py
@@ -10,17 +10,20 @@
 """
 
 import asyncio
+import hmac
 import time
 from typing import Annotated, Literal
 
-from fastapi import APIRouter, BackgroundTasks, Depends, Query
+from fastapi import APIRouter, BackgroundTasks, Depends, Header, Query
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from core.auth import get_current_user
+from core.config import settings
 from core.database import get_session
 from core.utils import setup_logger
 from models.user import User
+from services.document_telemetry import sanitize_source
 from services.search.classifier_service import ClassifierResult, classify
 from services.search.evidence_service import EvidenceItem, extract_evidence
 from services.search.fusion_service import DEFAULT_FUSION
@@ -367,6 +370,48 @@ def _build_ask_debug(
     )
 
 
+def _resolve_eval_identity(
+    x_source: str | None,
+    x_eval_case_id: str | None,
+    x_eval_token: str | None,
+) -> tuple[str, str | None]:
+    """X-Source/X-Eval-Case-Id 신뢰 검증 (Phase 3.5 fix2).
+
+    규칙:
+      - 기본값: source='document_server', eval_case_id=None
+      - X-Source=eval 또는 X-Eval-Case-Id 가 들어왔다면 eval claim 으로 간주
+      - eval claim 은 X-Eval-Token == settings.eval_runner_token 일 때만 수용
+        (constant-time compare, env 미설정 시 항상 거부)
+      - 거부 시: 헤더 무시 + warning log + source=sanitize(non-eval) / eval_case_id=None
+      - 통과 시: source='eval', eval_case_id=x_eval_case_id
+
+    반환: (source, eval_case_id)
+    """
+    claimed_source = sanitize_source(x_source)
+    is_eval_claim = (claimed_source == "eval") or bool(x_eval_case_id)
+    if not is_eval_claim:
+        # 일반 호출 — eval_case_id 강제 None (source != 'eval' 이면 case_id 의미 없음)
+        return claimed_source, None
+
+    # eval claim — token 검증
+    expected = settings.eval_runner_token
+    presented = x_eval_token or ""
+    token_valid = bool(expected) and hmac.compare_digest(presented, expected)
+    if not token_valid:
+        logger.warning(
+            "eval header rejected: source=%s case_id=%s token_present=%s expected_set=%s",
+            x_source, x_eval_case_id, bool(x_eval_token), bool(expected),
+        )
+        # 일반 호출로 강등 — source='eval' 주장은 무시, case_id 도 무시
+        # claimed_source 가 'eval' 이면 default 'document_server' 로
+        if claimed_source == "eval":
+            return "document_server", None
+        return claimed_source, None
+
+    # token OK — eval 라벨 수용
+    return "eval", x_eval_case_id
+
+
 @router.get("/ask", response_model=AskResponse)
 async def ask(
     q: str,
@@ -375,14 +420,24 @@ async def ask(
     background_tasks: BackgroundTasks,
     limit: int = Query(10, ge=1, le=20, description="synthesis 입력 상한"),
     debug: bool = Query(False, description="evidence/synthesis 중간 상태 노출"),
+    x_source: Annotated[str | None, Header(alias="X-Source")] = None,
+    x_eval_case_id: Annotated[str | None, Header(alias="X-Eval-Case-Id")] = None,
+    x_eval_token: Annotated[str | None, Header(alias="X-Eval-Token")] = None,
 ):
     """근거 기반 AI 답변 (Phase 3.5a).
 
     Phase 3.3 기반 + classifier parallel + refusal gate + grounding re-gate.
     실패 경로에서도 `results` 는 항상 반환.
+
+    Phase 3.5 calibration trust boundary (fix2):
+      - X-Source / X-Eval-Case-Id 는 X-Eval-Token 이 EVAL_RUNNER_TOKEN 와 일치하는
+        trusted internal eval runner 에서만 수용된다.
+      - 일반 client 의 X-Source=eval 시도는 무시되고 source='document_server' 로 강제.
+      - source != 'eval' 이면 eval_case_id 항상 None.
     """
     t_total = time.perf_counter()
     defense_log: dict = {}  # per-layer flag snapshot
+    source, eval_case_id = _resolve_eval_identity(x_source, x_eval_case_id, x_eval_token)
 
     # 1. 검색 파이프라인
     pr = await run_search(
@@ -500,6 +555,9 @@ async def ask(
             missing_aspects=classifier_result.missing_aspects or None,
             model_name=resolve_primary_model(),
             prompt_version=ASK_PROMPT_VERSION,
+            # Phase 3.5 calibration
+            source=source,
+            eval_case_id=eval_case_id,
         )
         debug_obj = None
         if debug:
@@ -697,6 +755,9 @@ async def ask(
         missing_aspects=missing_aspects,
         model_name=resolve_primary_model(),
         prompt_version=ASK_PROMPT_VERSION,
+        # Phase 3.5 calibration
+        source=source,
+        eval_case_id=eval_case_id,
     )
 
     debug_obj = None
diff --git a/app/core/config.py b/app/core/config.py
index 49ba5b8..4169498 100644
--- a/app/core/config.py
+++ b/app/core/config.py
@@ -45,6 +45,10 @@ class Settings(BaseModel):
     jwt_secret: str = ""
     totp_secret: str = ""
 
+    # Phase 3.5: eval runner shared secret — X-Source=eval / X-Eval-Case-Id 헤더 신뢰 검증.
+    # 비어있으면 모든 eval 헤더 거부 (부재 = 비활성).
+    eval_runner_token: str = ""
+
     # kordoc
     kordoc_endpoint: str = "http://kordoc-service:3100"
 
@@ -62,6 +66,7 @@ def load_settings() -> Settings:
     database_url = os.getenv("DATABASE_URL", "")
     jwt_secret = os.getenv("JWT_SECRET", "")
     totp_secret = os.getenv("TOTP_SECRET", "")
+    eval_runner_token = os.getenv("EVAL_RUNNER_TOKEN", "")
     kordoc_endpoint = os.getenv("KORDOC_ENDPOINT", "http://kordoc-service:3100")
     ocr_endpoint = os.getenv("OCR_ENDPOINT", "http://ocr-service:3200")
 
@@ -113,6 +118,7 @@ def load_settings() -> Settings:
         nas_pkm_root=nas_pkm,
         jwt_secret=jwt_secret,
         totp_secret=totp_secret,
+        eval_runner_token=eval_runner_token,
         kordoc_endpoint=kordoc_endpoint,
         ocr_endpoint=ocr_endpoint,
         taxonomy=taxonomy,
diff --git a/app/models/ask_event.py b/app/models/ask_event.py
index b70c061..1f5e393 100644
--- a/app/models/ask_event.py
+++ b/app/models/ask_event.py
@@ -39,6 +39,10 @@ class AskEvent(Base):
     missing_aspects: Mapped[list[Any] | None] = mapped_column(JSONB)
     model_name: Mapped[str | None] = mapped_column(Text)
     prompt_version: Mapped[str | None] = mapped_column(Text)
+    # Phase 3.5 calibration: eval/production 분리 + golden join 키
+    # 138~141 단계: nullable. 142 적용 후 source 는 NOT NULL (DB 강제, 앱은 항상 채움).
+    source: Mapped[str | None] = mapped_column(Text)
+    eval_case_id: Mapped[str | None] = mapped_column(Text)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=datetime.now, nullable=False
     )
diff --git a/app/services/search_telemetry.py b/app/services/search_telemetry.py
index 178cc6f..1dbb02e 100644
--- a/app/services/search_telemetry.py
+++ b/app/services/search_telemetry.py
@@ -333,6 +333,9 @@ async def record_ask_event(
     missing_aspects: list[str] | None = None,
     model_name: str | None = None,
     prompt_version: str | None = None,
+    # Phase 3.5 calibration: source 분리 + golden join
+    source: str | None = None,
+    eval_case_id: str | None = None,
 ) -> None:
     """ask_events INSERT. background task에서 호출 — 에러 삼킴.
 
@@ -341,6 +344,10 @@ async def record_ask_event(
       - covered_aspects / missing_aspects: classifier 결과 그대로
       - model_name: resolve_primary_model() 또는 호출사이트 명시
       - prompt_version: ASK_PROMPT_VERSION 상수
+
+    Phase 3.5 calibration:
+      - source: sanitize_source(X-Source 헤더) — eval/ui_search/ui_detail/...
+      - eval_case_id: X-Eval-Case-Id 헤더 (eval 호출만 채움)
     """
     try:
         async with async_session() as session:
@@ -364,6 +371,8 @@ async def record_ask_event(
                 missing_aspects=missing_aspects,
                 model_name=model_name,
                 prompt_version=prompt_version,
+                source=source,
+                eval_case_id=eval_case_id,
             )
             session.add(row)
             await session.commit()
diff --git a/credentials.env.example b/credentials.env.example
index 53fa237..efcf7cf 100644
--- a/credentials.env.example
+++ b/credentials.env.example
@@ -50,3 +50,10 @@ NYT_API_KEY=
 
 # ─── 국가법령정보센터 (법령 모니터링) ───
 LAW_OC=
+
+# ─── Phase 3.5 fix2: eval runner shared secret ───
+# /ask 엔드포인트의 X-Source=eval / X-Eval-Case-Id 헤더 신뢰 검증 토큰.
+# 비어있거나 클라이언트 X-Eval-Token 와 불일치 시 eval 헤더 거부 (warning log + source='document_server' 강등).
+# 충분히 긴 random secret 권장 (예: openssl rand -hex 32).
+# scripts/run_eval_ask.py runner 가 동일 값을 X-Eval-Token 헤더로 전송해야 eval telemetry 적재됨.
+EVAL_RUNNER_TOKEN=
diff --git a/migrations/138_ask_events_source_and_eval_case_id.sql b/migrations/138_ask_events_source_and_eval_case_id.sql
new file mode 100644
index 0000000..4c39a11
--- /dev/null
+++ b/migrations/138_ask_events_source_and_eval_case_id.sql
@@ -0,0 +1 @@
+ALTER TABLE ask_events ADD COLUMN IF NOT EXISTS source TEXT DEFAULT 'document_server', ADD COLUMN IF NOT EXISTS eval_case_id TEXT
diff --git a/migrations/139_ask_events_source_created_idx.sql b/migrations/139_ask_events_source_created_idx.sql
new file mode 100644
index 0000000..b3d1cd0
--- /dev/null
+++ b/migrations/139_ask_events_source_created_idx.sql
@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_ask_events_source_created ON ask_events(source, created_at DESC)
diff --git a/migrations/140_ask_events_eval_case_id_idx.sql b/migrations/140_ask_events_eval_case_id_idx.sql
new file mode 100644
index 0000000..eac9d16
--- /dev/null
+++ b/migrations/140_ask_events_eval_case_id_idx.sql
@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_ask_events_eval_case_id ON ask_events(eval_case_id) WHERE eval_case_id IS NOT NULL
diff --git a/migrations/141_ask_events_source_backfill.sql b/migrations/141_ask_events_source_backfill.sql
new file mode 100644
index 0000000..914dbe4
--- /dev/null
+++ b/migrations/141_ask_events_source_backfill.sql
@@ -0,0 +1 @@
+UPDATE ask_events SET source = 'document_server' WHERE source IS NULL
diff --git a/migrations/142_ask_events_source_notnull.sql b/migrations/142_ask_events_source_notnull.sql
new file mode 100644
index 0000000..1fab609
--- /dev/null
+++ b/migrations/142_ask_events_source_notnull.sql
@@ -0,0 +1 @@
+ALTER TABLE ask_events ALTER COLUMN source SET NOT NULL
diff --git a/tests/test_ask_eval_auth.py b/tests/test_ask_eval_auth.py
new file mode 100644
index 0000000..0492d21
--- /dev/null
+++ b/tests/test_ask_eval_auth.py
@@ -0,0 +1,92 @@
+"""Phase 3.5 fix2: /ask 의 X-Source / X-Eval-Case-Id trust boundary.
+
+`_resolve_eval_identity()` 단위 테스트.
+- token 없음/틀림 + X-Source=eval → source='document_server', eval_case_id=None
+- token 일치 + X-Source=eval + X-Eval-Case-Id=case_xxx → ('eval', 'case_xxx')
+- token 틀림 + X-Eval-Case-Id 만 (X-Source 미지정) → eval_case_id=None
+- 일반 호출 (X-Source=ui_search, no eval headers) → ('ui_search', None)
+- env 미설정 (eval_runner_token='') 시 모든 eval claim 거부
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "app"))
+
+import pytest
+
+
+@pytest.fixture
+def resolve_with_token(monkeypatch):
+    """settings.eval_runner_token 을 monkey-patch 해서 _resolve_eval_identity 테스트."""
+    def _make(token: str):
+        from core import config as cfg_mod
+        from api import search as search_mod
+        # 두 모듈 모두에서 settings 객체 참조하므로 직접 attr 변경
+        monkeypatch.setattr(search_mod.settings, "eval_runner_token", token)
+        return search_mod._resolve_eval_identity
+    return _make
+
+
+def test_no_token_no_eval_headers_default(resolve_with_token):
+    """일반 호출 — eval 헤더 없음, source 기본값."""
+    resolve = resolve_with_token("secret123")
+    assert resolve(None, None, None) == ("document_server", None)
+
+
+def test_normal_source_with_token(resolve_with_token):
+    """ui_search 호출 — eval 클레임 아님이라 token 무관."""
+    resolve = resolve_with_token("secret123")
+    assert resolve("ui_search", None, None) == ("ui_search", None)
+
+
+def test_eval_claim_no_token_rejected(resolve_with_token):
+    """X-Source=eval 인데 token 없음 → 거부, source='document_server'."""
+    resolve = resolve_with_token("secret123")
+    assert resolve("eval", "case_001", None) == ("document_server", None)
+
+
+def test_eval_claim_wrong_token_rejected(resolve_with_token):
+    """token 틀림 → 거부."""
+    resolve = resolve_with_token("secret123")
+    assert resolve("eval", "case_001", "wrong_token") == ("document_server", None)
+
+
+def test_eval_claim_correct_token_accepted(resolve_with_token):
+    """token 일치 → 'eval' source + case_id 적재."""
+    resolve = resolve_with_token("secret123")
+    assert resolve("eval", "case_001", "secret123") == ("eval", "case_001")
+
+
+def test_eval_case_id_only_no_source_no_token(resolve_with_token):
+    """X-Eval-Case-Id 만 있고 token 없음 → 거부, case_id=None."""
+    resolve = resolve_with_token("secret123")
+    assert resolve(None, "case_001", None) == ("document_server", None)
+
+
+def test_eval_case_id_only_wrong_token(resolve_with_token):
+    """X-Eval-Case-Id 만 + token 틀림 → 거부."""
+    resolve = resolve_with_token("secret123")
+    assert resolve(None, "case_001", "wrong") == ("document_server", None)
+
+
+def test_env_unset_rejects_even_correct_format(resolve_with_token):
+    """settings.eval_runner_token='' 인 환경 → 모든 eval 클레임 거부."""
+    resolve = resolve_with_token("")
+    # token 헤더가 와도 server side 가 비어있으면 거부 (constant-time False)
+    assert resolve("eval", "case_001", "") == ("document_server", None)
+    assert resolve("eval", "case_001", "anything") == ("document_server", None)
+
+
+def test_non_eval_source_forces_case_id_none(resolve_with_token):
+    """X-Source=ui_detail + X-Eval-Case-Id (실수로 같이 보냄) → case_id=None.
+
+    eval claim 아님 (source != 'eval' 이고 case_id 가 fallback 으로 eval claim 트리거)
+    이지만 source claim 이 명시적으로 non-eval 이라 token 검증 후 case_id None.
+    """
+    resolve = resolve_with_token("secret123")
+    # case_id 가 있으면 eval claim 으로 처리됨 → token 없으면 거부 → ('ui_detail' 클레임,
+    # 하지만 거부 분기에서 claimed_source != 'eval' 이라 그대로 'ui_detail' 반환, case_id=None)
+    assert resolve("ui_detail", "case_001", None) == ("ui_detail", None)