fix(acp-snapshot): stabilize /private prefix and spill paths in snapshots

macOS symlinks /tmp → /private/tmp; when the session header cwd lacks
the /private prefix but fs tools resolve symlinks, tool result paths
get the prefix and the cwd replacement never matches.  Strip the prefix
before cwd→{{cwd}} replacement so both forms normalize identically.

Also extend refreshFixtureReplacements to match spill paths by filename
suffix, preventing raw fixture churn when a refresh runs from a
different session directory.

packages/support/acp-snapshot/src/index.ts

@@ -33,6 +33,7 @@ export {
   type LaunchedAcpTestAgent,
 } from './launcher.ts'
 export {
+  extractSnapshotSpillPaths,
   normalizeSessionLog,
   normalizeStdout,
   scrubRequestHeaders,

packages/support/acp-snapshot/src/normalize.ts

@@ -30,6 +30,19 @@ const SNAPSHOT_SPILL_PATH_RE = new RegExp(
   'g',
 )
 
+/**
+ * Extract every snapshot-mode spill path from a session log, keyed by spill
+ * filename. Used by refresh write-back to keep spill paths stable across runs.
+ */
+export function extractSnapshotSpillPaths(content: string): Map<string, string> {
+  const result = new Map<string, string>()
+  for (const match of content.matchAll(SNAPSHOT_SPILL_PATH_RE)) {
+    const name = match[1]
+    if (name) result.set(name, match[0])
+  }
+  return result
+}
+
 /** Convert separators only inside generated path-bearing text markers. */
 function canonicalizeEmbeddedPaths(value: string): string {
   return value
@@ -61,6 +74,11 @@ function scrubString(value: string, ctx: NormalizeContext, cwdPathMode: CwdPathM
   let out = value
   // cwd first (longest, most specific), then explicit session ids, then any
   // residual UUID (covers ids that appear in places we didn't enumerate).
+  // macOS symlinks /tmp → /private/tmp and /var → /private/var. The session
+  // header cwd may be recorded without the /private prefix while fs tools
+  // resolve symlinks, producing paths with the prefix inside tool results.
+  // Strip the prefix before the cwd replacement so both forms match.
+  out = out.split(`/private${ctx.cwd}`).join(ctx.cwd)
   out = out.split(ctx.cwd).join(CWD)
   out = out.split(`/private${CWD}`).join(CWD)
   if (cwdPathMode === 'canonical') {

packages/support/acp-snapshot/src/suite.ts

@@ -23,6 +23,7 @@ import { type AgentUnderTest, type HarvestedLog, type InputScript, runScenario }
 import {
   type CwdPathMode,
   type NormalizeContext,
+  extractSnapshotSpillPaths,
   normalizeSessionLog,
   normalizeStdout,
   scrubRequestHeaders,
@@ -443,7 +444,7 @@ export function unknownToolCallIds(rawLog: string): string[] {
 }
 
 /**
- * Build the cross-log id/cwd replacements used by refresh write-back.
+ * Build the cross-log id/cwd/spill-path replacements used by refresh write-back.
  *
  * @param logs The freshly harvested logs, in fixture order.
  * @param fixtures The existing fixture contents, in matching order.
@@ -461,6 +462,16 @@ export function refreshFixtureReplacements(logs: HarvestedLog[], fixtures: strin
         replacements.push({ from, to })
       }
     }
+    // Stabilize snapshot spill paths: match by filename suffix so the raw
+    // fixture does not churn on every refresh from a different session run.
+    const freshSpills = extractSnapshotSpillPaths((logs[i] as HarvestedLog).content)
+    const existingSpills = extractSnapshotSpillPaths(fixtures[i] ?? '')
+    for (const [name, existingPath] of existingSpills) {
+      const freshPath = freshSpills.get(name)
+      if (freshPath !== undefined && freshPath !== existingPath) {
+        replacements.push({ from: freshPath, to: existingPath })
+      }
+    }
   }
   return replacements
 }

packages/support/acp-snapshot/tests/normalize.spec.ts

@@ -192,6 +192,21 @@ describe('normalizeSessionLog', () => {
     expect(out).not.toContain('/private{{spillLocator')
   })
 
+  it('scrubs macOS /private prefix on cwd-rooted fs tool result paths', () => {
+    const ev = JSON.stringify({
+      type: 'tool/result', seq: 2, time: 5,
+      data: {
+        content: [{
+          type: 'text',
+          text: `The file /private${ctx.cwd}/config.txt has been updated successfully.`,
+        }],
+      },
+    })
+    const out = normalizeSessionLog(`${header({ cwd: ctx.cwd })}\n${ev}\n`, ctx)
+    expect(out).toContain('{{cwd}}/config.txt')
+    expect(out).not.toContain('/private{{cwd}}')
+  })
+
   it('scrubs fixed snapshot spill paths', () => {
     const ev = JSON.stringify({
       type: 'tool/result', seq: 2, time: 5,