diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c49f858..7f4d271 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,6 +12,9 @@ jobs:
   build-and-test:
     runs-on: ubuntu-latest
     timeout-minutes: 15
+    env:
+      PY_FIXTURE_REF: dba101b662410c0ae0c85842cd78d8adb55f184b
+      PY_FIXTURE_CHECKOUT: ${{ runner.temp }}/context-compiler-source
 
     steps:
       - name: Checkout
@@ -27,12 +30,22 @@ jobs:
         run: npm ci
 
       - name: Checkout Python fixture source
-        # TS 0.5.2 fixtures include cases added after Python v0.5.2; pin to v0.6.0
-        # to keep fixture drift checks aligned with the current TS fixture corpus.
-        run: git clone --depth 1 --branch v0.6.0 https://github.com/rlippmann/context-compiler ../context-compiler
+        # Pin to an explicit Python commit SHA for deterministic fixture drift checks.
+        run: |
+          echo "Python fixture ref: ${PY_FIXTURE_REF}"
+          echo "Python checkout path: ${PY_FIXTURE_CHECKOUT}"
+          git init "${PY_FIXTURE_CHECKOUT}"
+          git -C "${PY_FIXTURE_CHECKOUT}" remote add origin https://github.com/rlippmann/context-compiler
+          git -C "${PY_FIXTURE_CHECKOUT}" fetch --depth 1 origin "${PY_FIXTURE_REF}"
+          git -C "${PY_FIXTURE_CHECKOUT}" checkout --detach FETCH_HEAD
+          echo "Resolved fixture source: ${PY_FIXTURE_CHECKOUT}/tests/fixtures/conformance"
 
       - name: Check fixture drift
-        run: npm run fixtures:check
+        env:
+          FIXTURES_SOURCE: ${{ runner.temp }}/context-compiler-source/tests/fixtures/conformance
+        run: |
+          echo "Using FIXTURES_SOURCE=${FIXTURES_SOURCE}"
+          npm run fixtures:check
 
       - name: Build
         run: npm run build
diff --git a/AGENTS.md b/AGENTS.md
index eca2d20..4f32f52 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -14,7 +14,7 @@ Behavior is defined by the upstream Python project:
 
 The following are authoritative:
 - Directive grammar specification
-- Fixture corpus under `tests/fixtures/v2`
+- Fixture corpus under `tests/fixtures/conformance`
 - Python engine behavior as exercised by those fixtures
 
 If behavior differs, the TypeScript implementation is incorrect.
@@ -78,10 +78,46 @@ A change is correct only if all fixtures pass.
 
 Do not modify fixtures to make tests pass.
 
-Fixture updates are allowed only when syncing from the authoritative Python source for the targeted compatibility line.
+Fixture updates must follow the process defined in 'Conformance fixtures' below.
 
 If synced fixtures introduce failures, fix TypeScript behavior rather than editing fixture expectations.
 
+## Conformance fixtures
+
+Conformance fixtures are sourced from the Python repository and define the canonical cross-port contract.
+
+### Rules
+
+- Do not hand-edit fixture JSON files.
+- Do not modify fixture JSON to make TS tests pass.
+- TS implementation must conform to the fixtures, not the reverse.
+- Fixture JSON files are read-only artifacts in this repository.
+
+### Updating fixtures
+
+When fixtures need to be updated:
+
+1. Obtain the Python source of truth (pinned commit or tag).
+2. Run the sync script with an explicit source:
+
+   FIXTURES_SOURCE=<python repo>/tests/fixtures/conformance npm run fixtures:sync
+
+3. Commit only the resulting changes.
+
+### Verification
+
+After syncing:
+
+npm test
+FIXTURES_SOURCE=<same python fixtures path> npm run fixtures:check
+
+### Constraints
+
+- FIXTURES_SOURCE must be explicitly provided; no implicit local fallback is allowed.
+- Do not copy/paste or manually edit fixture contents, even for single-file fixes.
+- If a fixture appears incorrect, fix it in the Python repo first, then re-sync.
+- Manual fixture JSON edits should be rejected in review.
+
 ## Test Coverage Expectations
 
 Before opening a PR, consider:
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index b945285..b08df59 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -68,19 +68,18 @@ All tests must pass before submitting changes.
 
 Python fixtures are the source of truth.
 
-Default source path:
+Conformance fixture policy:
 
-- `../context-compiler/tests/fixtures/v2`
+- Do not hand-edit fixture JSON files under `tests/fixtures/conformance`.
+- Update conformance fixtures only via `npm run fixtures:sync` with explicit `FIXTURES_SOURCE` pointing to the Python source of truth.
+- If fixture updates introduce test failures, update TypeScript implementation to conform to fixtures.
 
 Commands:
 
-- `npm run fixtures:sync` to copy fixtures from Python into `tests/fixtures/v2`
-- `npm run fixtures:check` to detect drift between local fixtures and Python fixtures
+- `FIXTURES_SOURCE=/path/to/context-compiler/tests/fixtures/conformance npm run fixtures:sync` to copy fixtures from Python into `tests/fixtures/conformance`
+- `FIXTURES_SOURCE=/path/to/context-compiler/tests/fixtures/conformance npm run fixtures:check` to detect drift between local fixtures and Python fixtures
 
-Optional override:
-
-- Set `FIXTURES_SOURCE` to use a different source path, for example:
-  - `FIXTURES_SOURCE=/path/to/context-compiler/tests/fixtures/v2 npm run fixtures:check`
+`FIXTURES_SOURCE` is required for both commands.
 
 ## Pull Requests
 
diff --git a/scripts/fixtures-check.sh b/scripts/fixtures-check.sh
index 8f3da2d..b845883 100755
--- a/scripts/fixtures-check.sh
+++ b/scripts/fixtures-check.sh
@@ -1,12 +1,20 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-SOURCE_DIR="${FIXTURES_SOURCE:-../context-compiler/tests/fixtures/v2}"
-TARGET_DIR="tests/fixtures/v2"
+if [[ -z "${FIXTURES_SOURCE:-}" ]]; then
+  echo "[fixtures:check] FIXTURES_SOURCE is required." >&2
+  echo "[fixtures:check] Example: FIXTURES_SOURCE=/path/to/context-compiler/tests/fixtures/conformance npm run fixtures:check" >&2
+  exit 1
+fi
+
+SOURCE_DIR="$FIXTURES_SOURCE"
+TARGET_DIR="tests/fixtures/conformance"
+
+echo "[fixtures:check] Using source fixture directory: $SOURCE_DIR"
 
 if [[ ! -d "$SOURCE_DIR" ]]; then
   echo "[fixtures:check] Source fixture directory not found: $SOURCE_DIR" >&2
-  echo "[fixtures:check] Set FIXTURES_SOURCE to override the default source path." >&2
+  echo "[fixtures:check] Set FIXTURES_SOURCE to a valid Python fixture source path." >&2
   exit 1
 fi
 
diff --git a/scripts/fixtures-sync.sh b/scripts/fixtures-sync.sh
index b7c9cf9..80e242f 100755
--- a/scripts/fixtures-sync.sh
+++ b/scripts/fixtures-sync.sh
@@ -1,12 +1,20 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-SOURCE_DIR="${FIXTURES_SOURCE:-../context-compiler/tests/fixtures/v2}"
-TARGET_DIR="tests/fixtures/v2"
+if [[ -z "${FIXTURES_SOURCE:-}" ]]; then
+  echo "[fixtures:sync] FIXTURES_SOURCE is required." >&2
+  echo "[fixtures:sync] Example: FIXTURES_SOURCE=/path/to/context-compiler/tests/fixtures/conformance npm run fixtures:sync" >&2
+  exit 1
+fi
+
+SOURCE_DIR="$FIXTURES_SOURCE"
+TARGET_DIR="tests/fixtures/conformance"
+
+echo "[fixtures:sync] Using source fixture directory: $SOURCE_DIR"
 
 if [[ ! -d "$SOURCE_DIR" ]]; then
   echo "[fixtures:sync] Source fixture directory not found: $SOURCE_DIR" >&2
-  echo "[fixtures:sync] Set FIXTURES_SOURCE to override the default source path." >&2
+  echo "[fixtures:sync] Set FIXTURES_SOURCE to a valid Python fixture source path." >&2
   exit 1
 fi
 
diff --git a/src/engine.ts b/src/engine.ts
index 3356934..4507981 100644
--- a/src/engine.ts
+++ b/src/engine.ts
@@ -235,18 +235,7 @@ class EngineImpl implements Engine {
       const oldState = this._state.policies[oldKey];
       const newState = this._state.policies[newKey];
       if (!Object.prototype.hasOwnProperty.call(this._state.policies, oldKey)) {
-        const promptLines = [
-          `No exact policy found for "${action.old_item}".`,
-          'Replacement requires an exact policy match.'
-        ];
-        const diagnosticHints = diagnosticPolicyContainsHints(this._state.policies, action.old_item);
-        if (diagnosticHints !== '') {
-          promptLines.push(`Existing policies containing that text: ${diagnosticHints}.`);
-          promptLines.push(`Confirm to use "${action.new_item}" and keep ${diagnosticHints}?`);
-        } else {
-          promptLines.push(`Confirm to use "${action.new_item}" and keep existing policies?`);
-        }
-        const prompt = promptLines.join('\n');
+        const prompt = `Did you mean to use "${action.new_item}" instead?`;
         this._pendingReplacement = { kind: 'use_only', new_item: action.new_item };
         this._pendingPrompt = prompt;
         return clarify(prompt);
diff --git a/tests/engine_state_immutability.test.ts b/tests/engine_state_immutability.test.ts
index 84042b3..04d2a74 100644
--- a/tests/engine_state_immutability.test.ts
+++ b/tests/engine_state_immutability.test.ts
@@ -105,7 +105,7 @@ describe('engine error-path regressions', () => {
     expectClarifyNoMutation(
       missingSource,
       'use kubectl instead of docker',
-      'No exact policy found for "docker".\nReplacement requires an exact policy match.\nConfirm to use "kubectl" and keep existing policies?'
+      'Did you mean to use "kubectl" instead?'
     );
 
     const oldIsProhibit = createEngine({ state: { premise: null, policies: { docker: 'prohibit' }, version: 2 } });
diff --git a/tests/fixtures/v2/step/001_set_premise_update.json b/tests/fixtures/conformance/step/001_set_premise_update.json
similarity index 100%
rename from tests/fixtures/v2/step/001_set_premise_update.json
rename to tests/fixtures/conformance/step/001_set_premise_update.json
diff --git a/tests/fixtures/v2/step/002_use_item_normalization.json b/tests/fixtures/conformance/step/002_use_item_normalization.json
similarity index 100%
rename from tests/fixtures/v2/step/002_use_item_normalization.json
rename to tests/fixtures/conformance/step/002_use_item_normalization.json
diff --git a/tests/fixtures/v2/step/003_conflict_prohibit_clarify.json b/tests/fixtures/conformance/step/003_conflict_prohibit_clarify.json
similarity index 100%
rename from tests/fixtures/v2/step/003_conflict_prohibit_clarify.json
rename to tests/fixtures/conformance/step/003_conflict_prohibit_clarify.json
diff --git a/tests/fixtures/v2/step/004_remove_policy_missing_idempotent_update.json b/tests/fixtures/conformance/step/004_remove_policy_missing_idempotent_update.json
similarity index 100%
rename from tests/fixtures/v2/step/004_remove_policy_missing_idempotent_update.json
rename to tests/fixtures/conformance/step/004_remove_policy_missing_idempotent_update.json
diff --git a/tests/fixtures/v2/step/005_exact_prefix_passthrough_leading_space.json b/tests/fixtures/conformance/step/005_exact_prefix_passthrough_leading_space.json
similarity index 100%
rename from tests/fixtures/v2/step/005_exact_prefix_passthrough_leading_space.json
rename to tests/fixtures/conformance/step/005_exact_prefix_passthrough_leading_space.json
diff --git a/tests/fixtures/v2/step/006_near_miss_set_premise_to.json b/tests/fixtures/conformance/step/006_near_miss_set_premise_to.json
similarity index 100%
rename from tests/fixtures/v2/step/006_near_miss_set_premise_to.json
rename to tests/fixtures/conformance/step/006_near_miss_set_premise_to.json
diff --git a/tests/fixtures/v2/step/007_near_miss_change_premise_missing_to.json b/tests/fixtures/conformance/step/007_near_miss_change_premise_missing_to.json
similarity index 100%
rename from tests/fixtures/v2/step/007_near_miss_change_premise_missing_to.json
rename to tests/fixtures/conformance/step/007_near_miss_change_premise_missing_to.json
diff --git a/tests/fixtures/v2/step/008_replace_missing_source_clarify_prompt.json b/tests/fixtures/conformance/step/008_replace_missing_source_clarify_prompt.json
similarity index 69%
rename from tests/fixtures/v2/step/008_replace_missing_source_clarify_prompt.json
rename to tests/fixtures/conformance/step/008_replace_missing_source_clarify_prompt.json
index 4b87a0b..d5f476d 100644
--- a/tests/fixtures/v2/step/008_replace_missing_source_clarify_prompt.json
+++ b/tests/fixtures/conformance/step/008_replace_missing_source_clarify_prompt.json
@@ -10,7 +10,7 @@
   "expected": {
     "decision": {
       "kind": "clarify",
-      "prompt_to_user": "No exact policy found for \"docker\".\nReplacement requires an exact policy match.\nConfirm to use \"kubectl\" and keep existing policies?",
+      "prompt_to_user": "Did you mean to use \"kubectl\" instead?",
       "state": null
     },
     "state": {
diff --git a/tests/fixtures/v2/step/009_pending_affirmative_normalized_token.json b/tests/fixtures/conformance/step/009_pending_affirmative_normalized_token.json
similarity index 100%
rename from tests/fixtures/v2/step/009_pending_affirmative_normalized_token.json
rename to tests/fixtures/conformance/step/009_pending_affirmative_normalized_token.json
diff --git a/tests/fixtures/v2/step/010_pending_negative_normalized_token.json b/tests/fixtures/conformance/step/010_pending_negative_normalized_token.json
similarity index 100%
rename from tests/fixtures/v2/step/010_pending_negative_normalized_token.json
rename to tests/fixtures/conformance/step/010_pending_negative_normalized_token.json
diff --git a/tests/fixtures/v2/step/011_pending_unmatched_reuses_prompt.json b/tests/fixtures/conformance/step/011_pending_unmatched_reuses_prompt.json
similarity index 100%
rename from tests/fixtures/v2/step/011_pending_unmatched_reuses_prompt.json
rename to tests/fixtures/conformance/step/011_pending_unmatched_reuses_prompt.json
diff --git a/tests/fixtures/v2/step/012_clear_premise_populated_update.json b/tests/fixtures/conformance/step/012_clear_premise_populated_update.json
similarity index 100%
rename from tests/fixtures/v2/step/012_clear_premise_populated_update.json
rename to tests/fixtures/conformance/step/012_clear_premise_populated_update.json
diff --git a/tests/fixtures/v2/step/013_clear_premise_already_null_update.json b/tests/fixtures/conformance/step/013_clear_premise_already_null_update.json
similarity index 100%
rename from tests/fixtures/v2/step/013_clear_premise_already_null_update.json
rename to tests/fixtures/conformance/step/013_clear_premise_already_null_update.json
diff --git a/tests/fixtures/v2/step/014_reset_policies_populated_update.json b/tests/fixtures/conformance/step/014_reset_policies_populated_update.json
similarity index 100%
rename from tests/fixtures/v2/step/014_reset_policies_populated_update.json
rename to tests/fixtures/conformance/step/014_reset_policies_populated_update.json
diff --git a/tests/fixtures/v2/step/015_reset_policies_already_empty_update.json b/tests/fixtures/conformance/step/015_reset_policies_already_empty_update.json
similarity index 100%
rename from tests/fixtures/v2/step/015_reset_policies_already_empty_update.json
rename to tests/fixtures/conformance/step/015_reset_policies_already_empty_update.json
diff --git a/tests/fixtures/v2/step/016_clear_state_populated_update.json b/tests/fixtures/conformance/step/016_clear_state_populated_update.json
similarity index 100%
rename from tests/fixtures/v2/step/016_clear_state_populated_update.json
rename to tests/fixtures/conformance/step/016_clear_state_populated_update.json
diff --git a/tests/fixtures/v2/step/017_clear_state_already_empty_update.json b/tests/fixtures/conformance/step/017_clear_state_already_empty_update.json
similarity index 100%
rename from tests/fixtures/v2/step/017_clear_state_already_empty_update.json
rename to tests/fixtures/conformance/step/017_clear_state_already_empty_update.json
diff --git a/tests/fixtures/v2/transcript/001_user_only_replay_state.json b/tests/fixtures/conformance/transcript/001_user_only_replay_state.json
similarity index 100%
rename from tests/fixtures/v2/transcript/001_user_only_replay_state.json
rename to tests/fixtures/conformance/transcript/001_user_only_replay_state.json
diff --git a/tests/fixtures/v2/transcript/002_non_string_user_content_ignored.json b/tests/fixtures/conformance/transcript/002_non_string_user_content_ignored.json
similarity index 100%
rename from tests/fixtures/v2/transcript/002_non_string_user_content_ignored.json
rename to tests/fixtures/conformance/transcript/002_non_string_user_content_ignored.json
diff --git a/tests/fixtures/v2/transcript/003_stops_at_first_clarify_later_yes.json b/tests/fixtures/conformance/transcript/003_stops_at_first_clarify_later_yes.json
similarity index 100%
rename from tests/fixtures/v2/transcript/003_stops_at_first_clarify_later_yes.json
rename to tests/fixtures/conformance/transcript/003_stops_at_first_clarify_later_yes.json
diff --git a/tests/fixtures/v2/transcript/004_stops_at_first_clarify_later_no.json b/tests/fixtures/conformance/transcript/004_stops_at_first_clarify_later_no.json
similarity index 100%
rename from tests/fixtures/v2/transcript/004_stops_at_first_clarify_later_no.json
rename to tests/fixtures/conformance/transcript/004_stops_at_first_clarify_later_no.json
diff --git a/tests/harness/fixtures.ts b/tests/harness/fixtures.ts
index 6d58368..94e73dc 100644
--- a/tests/harness/fixtures.ts
+++ b/tests/harness/fixtures.ts
@@ -40,7 +40,7 @@ export interface NamedFixture<T> {
   payload: T;
 }
 
-const FIXTURE_ROOT = resolve(process.cwd(), 'tests', 'fixtures', 'v2');
+const FIXTURE_ROOT = resolve(process.cwd(), 'tests', 'fixtures', 'conformance');
 
 async function listJsonFilesRecursive(dir: string): Promise<string[]> {
   const entries = await readdir(dir, { withFileTypes: true });
diff --git a/tests/step-fixtures.test.ts b/tests/step-fixtures.test.ts
index 0d01128..3024a96 100644
--- a/tests/step-fixtures.test.ts
+++ b/tests/step-fixtures.test.ts
@@ -4,7 +4,7 @@ import { loadStepFixtures } from './harness/fixtures.js';
 
 const fixtures = await loadStepFixtures();
 
-describe('step fixtures (v2)', () => {
+describe('step fixtures (conformance)', () => {
   for (const fixture of fixtures) {
     it(fixture.name, () => {
       expect(fixture.payload.kind).toBe('step');
diff --git a/tests/transcript-fixtures.test.ts b/tests/transcript-fixtures.test.ts
index 7f9b64d..2ab476e 100644
--- a/tests/transcript-fixtures.test.ts
+++ b/tests/transcript-fixtures.test.ts
@@ -4,7 +4,7 @@ import { loadTranscriptFixtures } from './harness/fixtures.js';
 
 const fixtures = await loadTranscriptFixtures();
 
-describe('transcript fixtures (v2)', () => {
+describe('transcript fixtures (conformance)', () => {
   for (const fixture of fixtures) {
     it(fixture.name, () => {
       expect(fixture.payload.kind).toBe('transcript');