fix(coder): harden edit-apply pipeline against block duplication

Root cause: two proven corruption mechanisms — (M1) non-idempotent apply
stamped the same block N times when a quantized model re-emitted the same
edit_file call or a turn was retried; (M2) Levenshtein tier 4 was fail-open
with no uniqueness guard, silently splicing into the wrong location.

Fixes applied at every layer of the pipeline:

Matcher (fuzzy-match.ts): raise SIMILARITY_THRESHOLD 0.66 → 0.85; add
AMBIGUITY_EPSILON uniqueness guard — two windows within 0.05 of the top
score → ambiguous, not a guess; add block-anchor gate (≥3-line needles
require first+last line exact match before a window is scored).

Edit planner (pending_changes.ts): extract planEdit() as a pure function;
idempotency guards detect already-applied states (anchored insert re-stamp,
old-gone-but-new-present); findPendingDuplicate() collapses identical
pending rows at queue time so M1 never reaches applyOne.

Atomic writes (pending_changes.ts): temp-file + rename on the same
filesystem so a crash can't leave a half-written source file; realpath()
first so symlinks survive the rename.

Per-file mutex (pending_changes.ts): withFileLock() serializes concurrent
read-modify-write on the same path via a chained-Promise Map.

EOL preservation (pending_changes.ts): normalize CRLF → LF for matching,
restore native line ending on write so Windows-style files stay clean.

Context isolation (inference_context.ts): replace module-level singleton
with AsyncLocalStorage so concurrent inference runs (arena parallel
dispatch, dispatcher poll racing a user message) each get their own
scoped context with no clobbering.

Tests: plan-edit.test.ts (pure planEdit unit tests), extended fuzzy-match
and pending_changes_integration suites, ALS isolation test that proves
overlapping runs get correct session IDs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/coder/src/services/__tests__/fuzzy-match.test.ts

@@ -161,6 +161,52 @@ describe('locateMatch — strategy 4: Levenshtein', () => {
   });
 });
 
+describe('locateMatch — strategy 4: fail-closed on ambiguity (corruption guard)', () => {
+  it('refuses (ambiguous) when two equally-similar anchored blocks both clear the bar', () => {
+    // The repetitive-file case that duplicated blocks: two blocks share the same
+    // first+last anchor lines and their middle lines are EQUALLY similar to the
+    // (drifted) needle. Tier 4 must refuse rather than splice over one of them.
+    const content = [
+      'const x = {',
+      '  total = aa;',
+      '};',
+      'const x = {',
+      '  total = bb;',
+      '};',
+    ].join('\n');
+    const needle = ['const x = {', '  total = ab;', '};'].join('\n');
+    const result = locateMatch(content, needle);
+    expect(result.kind).toBe('ambiguous');
+  });
+
+  it('refuses a below-threshold near-miss that the old 0.66 floor would have spliced', () => {
+    // ~0.7 similar: under the raised 0.85 floor this is now not_found, so the
+    // caller surfaces a correctable error instead of corrupting the file.
+    const content = 'const grandTotalAmount = a + b;\n';
+    const needle = 'const totalValue = a + b;';
+    const result = locateMatch(content, needle);
+    expect(result).toEqual({ kind: 'not_found' });
+  });
+
+  it('still matches a single genuine high-similarity drift uniquely', () => {
+    const content = 'const total = sum + tax;\n';
+    const needle = 'const totals = sum + tax;'; // one-char typo, ~0.96
+    const result = locateMatch(content, needle);
+    expect(result.kind).toBe('fuzzy');
+    const { start, end } = span(result);
+    expect(content.slice(start, end)).toBe('const total = sum + tax;');
+  });
+
+  it('requires an exact first+last line anchor for multi-line needles', () => {
+    // First line drifted too far to anchor → no window is scored → not_found,
+    // even though the middle lines are identical.
+    const content = ['function compute() {', '  return a + b;', '  return done;', '}'].join('\n');
+    const needle = ['totally different opener', '  return a + b;', '}'].join('\n');
+    const result = locateMatch(content, needle);
+    expect(result).toEqual({ kind: 'not_found' });
+  });
+});
+
 describe('locateMatch — edge cases', () => {
   it('returns not_found for an empty needle', () => {
     expect(locateMatch('anything', '')).toEqual({ kind: 'not_found' });

apps/coder/src/services/__tests__/pending_changes_integration.test.ts

@@ -83,6 +83,53 @@ describe.runIf(!!process.env.DATABASE_URL)('pending_changes integration', () =>
     expect(existsSync(resolve(testDir, 'deleteme.txt'))).toBe(false);
   });
 
+  it('re-emitted identical edits dedupe at queue and never duplicate on apply', async () => {
+    // Regression: the 2-3x block-stamping corruption. An anchored insert queued
+    // three times (a local model re-emitting the same tool call) must collapse to
+    // ONE pending row and apply exactly once.
+    await queueCreate(sql, testSessionId, null, 'dup.js', '<script>\nrender();\n', projectRoot)
+      .then((c) => applyOne(sql, c.id, projectRoot));
+
+    const oldStr = '<script>';
+    const newStr = '<script>\nconst recordFormats = ["gif"];';
+    const a = await queueEdit(sql, testSessionId, null, 'dup.js', oldStr, newStr, projectRoot);
+    const b = await queueEdit(sql, testSessionId, null, 'dup.js', oldStr, newStr, projectRoot);
+    const c = await queueEdit(sql, testSessionId, null, 'dup.js', oldStr, newStr, projectRoot);
+    // All three calls return the SAME pending row (deduped).
+    expect(b.id).toBe(a.id);
+    expect(c.id).toBe(a.id);
+
+    await applyOne(sql, a.id, projectRoot);
+    let content = await readFile(resolve(testDir, 'dup.js'), 'utf8');
+    expect((content.match(/const recordFormats/g) || []).length).toBe(1);
+
+    // Even a fresh, separately-queued identical edit re-applied is a no-op, not a stamp.
+    const again = await queueEdit(sql, testSessionId, null, 'dup.js', oldStr, newStr, projectRoot);
+    const res = await applyOne(sql, again.id, projectRoot);
+    expect(res.success).toBe(true);
+    content = await readFile(resolve(testDir, 'dup.js'), 'utf8');
+    expect((content.match(/const recordFormats/g) || []).length).toBe(1);
+  });
+
+  it('preserves CRLF line endings on edit', async () => {
+    await queueCreate(sql, testSessionId, null, 'crlf.txt', 'line one\r\nline two\r\nline three\r\n', projectRoot)
+      .then((c) => applyOne(sql, c.id, projectRoot));
+    const edit = await queueEdit(sql, testSessionId, null, 'crlf.txt', 'line two', 'line TWO', projectRoot);
+    const res = await applyOne(sql, edit.id, projectRoot);
+    expect(res.success).toBe(true);
+    const content = await readFile(resolve(testDir, 'crlf.txt'), 'utf8');
+    expect(content).toBe('line one\r\nline TWO\r\nline three\r\n');
+  });
+
+  it('refuses an edit that matches multiple locations instead of corrupting', async () => {
+    await queueCreate(sql, testSessionId, null, 'ambig.js', 'x=1;\ny=2;\nx=1;\n', projectRoot)
+      .then((ch) => applyOne(sql, ch.id, projectRoot));
+    const edit = await queueEdit(sql, testSessionId, null, 'ambig.js', 'x=1;', 'x=9;', projectRoot);
+    const res = await applyOne(sql, edit.id, projectRoot);
+    expect(res.success).toBe(false);
+    expect(res.error).toMatch(/matches 2 locations/);
+  });
+
   it('rewindOne → verify reverted', async () => {
     // Setup: create and apply a file
     const createChange = await queueCreate(sql, testSessionId, null, 'rewindable.txt', 'initial', projectRoot);

apps/coder/src/services/__tests__/plan-edit.test.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect } from 'vitest';
+import { planEdit } from '../pending_changes.js';
+
+// planEdit is the pure core of applyOne's edit splice. These tests pin the
+// idempotency guards that stop the "block stamped 2-3x" corruption: applying the
+// same queued edit more than once must be a no-op, never a duplicate.
+
+describe('planEdit — normal application', () => {
+  it('applies a unique exact edit', () => {
+    const content = 'a\nfoo\nb\n';
+    const plan = planEdit(content, 'foo', 'bar');
+    expect(plan).toEqual({ kind: 'apply', updated: 'a\nbar\nb\n' });
+  });
+
+  it('reports ambiguous when old_string occurs more than once', () => {
+    const content = 'foo\nx\nfoo\n';
+    const plan = planEdit(content, 'foo', 'bar');
+    expect(plan).toEqual({ kind: 'ambiguous', count: 2 });
+  });
+
+  it('reports not_found when old_string is absent and new is not present', () => {
+    const content = 'alpha\nbeta\n';
+    const plan = planEdit(content, 'gamma that is clearly nowhere', 'delta');
+    expect(plan).toEqual({ kind: 'not_found' });
+  });
+});
+
+describe('planEdit — idempotency (the corruption guard)', () => {
+  it('treats a re-applied anchored insert as already-applied (no duplicate)', () => {
+    // The exact mechanism that tripled `const recordFormats` in settings.html:
+    // an anchored insert (old=anchor, new=anchor+block) where the anchor still
+    // matches uniquely after the first apply.
+    const oldStr = '<script>';
+    const newStr = '<script>\nconst recordFormats = ["gif","mp4"];';
+    const before = '<script>\nfunction render() {}\n</script>\n';
+
+    const first = planEdit(before, oldStr, newStr);
+    expect(first.kind).toBe('apply');
+    const after = first.kind === 'apply' ? first.updated : '';
+    expect((after.match(/const recordFormats/g) || []).length).toBe(1);
+
+    // Re-applying the identical edit to the already-edited content is a no-op.
+    const second = planEdit(after, oldStr, newStr);
+    expect(second).toEqual({ kind: 'noop', reason: 'already-applied' });
+  });
+
+  it('treats an edit whose old_string is gone but new_string is present as already-applied', () => {
+    const content = 'const total = sum + tax;\n';
+    const plan = planEdit(content, 'const subtotal = sum;', 'const total = sum + tax;');
+    expect(plan).toEqual({ kind: 'noop', reason: 'already-applied' });
+  });
+
+  it('treats a no-change splice as a noop', () => {
+    const content = 'a\nfoo\nb\n';
+    const plan = planEdit(content, 'foo', 'foo');
+    expect(plan).toEqual({ kind: 'noop', reason: 'identical' });
+  });
+
+  it('does not duplicate across three repeated applications', () => {
+    const oldStr = 'function f() {';
+    const newStr = 'function f() {\n  const x = 1;';
+    let content = 'function f() {\n  return x;\n}\n';
+    for (let i = 0; i < 3; i++) {
+      const plan = planEdit(content, oldStr, newStr);
+      if (plan.kind === 'apply') content = plan.updated;
+    }
+    expect((content.match(/const x = 1;/g) || []).length).toBe(1);
+  });
+});