diff --git a/bin/gstack-gbrain-sync.ts b/bin/gstack-gbrain-sync.ts
index 4fc658ac4..0f0263f12 100644
--- a/bin/gstack-gbrain-sync.ts
+++ b/bin/gstack-gbrain-sync.ts
@@ -32,7 +32,7 @@
 import { existsSync, statSync, mkdirSync, writeFileSync, readFileSync, unlinkSync, renameSync } from "fs";
 import { join, dirname } from "path";
 import { execSync, spawnSync } from "child_process";
-import { homedir } from "os";
+import { homedir, hostname } from "os";
 import { createHash } from "crypto";
 
 import "../lib/conductor-env-shim";
@@ -161,30 +161,35 @@ function originUrl(): string | null {
 }
 
 /**
- * Derive a worktree-aware source id for the cwd code corpus.
+ * Derive a host- and worktree-aware source id for the cwd code corpus.
  *
- * Pattern: `gstack-code-<slug>-<pathhash8>` where slug comes from origin
- * (org/repo) and pathhash8 is the first 8 hex chars of sha1(absolute repo
- * path). The pathhash8 is what makes Conductor worktrees of the same repo
- * coexist as separate sources in the same gbrain DB instead of stomping on
- * each other.
+ * Pattern: `gstack-code-<slug>-<hostpathhash8>` where slug comes from origin
+ * (org/repo) and hostpathhash8 is the first 8 hex chars of
+ * sha1(`${hostname}::${absolute repo path}`). Folding hostname into the hash
+ * keeps Conductor worktrees of the same repo as distinct sources on one host
+ * AND keeps two machines that share an absolute layout (e.g. chezmoi-managed
+ * home dirs against a federated brain) from colliding on each other.
  *
  * Falls back to the repo basename when there is no origin (local repo).
  *
+ * `GSTACK_HOSTNAME` env override is honored for deterministic tests; in
+ * production paths it is unset and `os.hostname()` is used.
+ *
  * gbrain enforces source ids to be 1-32 lowercase alnum chars with
  * optional interior hyphens. `constrainSourceId` handles the 32-char cap
  * with a hashed-tail fallback when the combined slug exceeds budget.
  */
 function deriveCodeSourceId(repoPath: string): string {
-  const pathHash = createHash("sha1").update(repoPath).digest("hex").slice(0, 8);
+  const host = process.env.GSTACK_HOSTNAME || hostname();
+  const hostPathHash = createHash("sha1").update(`${host}::${repoPath}`).digest("hex").slice(0, 8);
   const remote = canonicalizeRemote(originUrl());
   if (remote) {
     const segs = remote.split("/").filter(Boolean);
     const slugSource = segs.slice(-2).join("-");
-    return constrainSourceId("gstack-code", `${slugSource}-${pathHash}`);
+    return constrainSourceId("gstack-code", `${slugSource}-${hostPathHash}`);
   }
   const base = repoPath.split("/").pop() || "repo";
-  return constrainSourceId("gstack-code", `${base}-${pathHash}`);
+  return constrainSourceId("gstack-code", `${base}-${hostPathHash}`);
 }
 
 /**
@@ -208,6 +213,172 @@ function deriveLegacyCodeSourceId(repoPath: string): string {
   return constrainSourceId("gstack-code", base);
 }
 
+/**
+ * Pre-#1468 path-only-hash source id, kept for hostname-fold migration only.
+ *
+ * Before the hostname fold, `deriveCodeSourceId` hashed only the absolute
+ * repo path: `gstack-code-<slug>-<sha1(path).slice(0,8)>`. After #1468 the
+ * hash key is `${hostname}::${path}`, so every existing user's brain has a
+ * legacy id that no longer matches what `deriveCodeSourceId` produces. We
+ * detect this form once, attempt rename-in-place if the gbrain CLI supports
+ * `sources rename`, and otherwise clean up after the new source successfully
+ * syncs. Distinct from `deriveLegacyCodeSourceId` (pre-pathhash v1.x form);
+ * both probes run.
+ */
+export function derivePathOnlyHashLegacyId(repoPath: string): string {
+  const pathHash = createHash("sha1").update(repoPath).digest("hex").slice(0, 8);
+  const remote = canonicalizeRemote(originUrl());
+  if (remote) {
+    const segs = remote.split("/").filter(Boolean);
+    const slugSource = segs.slice(-2).join("-");
+    return constrainSourceId("gstack-code", `${slugSource}-${pathHash}`);
+  }
+  const base = repoPath.split("/").pop() || "repo";
+  return constrainSourceId("gstack-code", `${base}-${pathHash}`);
+}
+
+/**
+ * Feature-check whether the installed gbrain CLI ships `sources rename <old> <new>`.
+ *
+ * Per the v1.40.0.0 design review: probing `gbrain sources rename --help` and
+ * matching for the exact argument shape catches the case where gbrain's
+ * `sources` parent help mentions a `rename` subcommand but the CLI doesn't
+ * accept the `<old> <new>` form (or vice versa). Cached for the lifetime
+ * of the process. As of gbrain 0.35.0.0 this command does not exist, so the
+ * function returns false and the migration path falls back to register-new
+ * + sync-OK + remove-old.
+ */
+let _gbrainSupportsRenameCache: boolean | null = null;
+export function _resetGbrainSupportsRenameCache(): void {
+  _gbrainSupportsRenameCache = null;
+}
+function gbrainSupportsSourcesRename(env?: NodeJS.ProcessEnv): boolean {
+  if (_gbrainSupportsRenameCache !== null) return _gbrainSupportsRenameCache;
+  try {
+    const r = spawnSync("gbrain", ["sources", "rename", "--help"], {
+      encoding: "utf-8",
+      timeout: 5_000,
+      stdio: ["ignore", "pipe", "pipe"],
+      env: env || process.env,
+    });
+    const out = `${r.stdout || ""}\n${r.stderr || ""}`;
+    // Match the exact argument shape: `rename <old> <new>` (with literal
+    // angle brackets in usage strings) or `rename OLD NEW`.
+    const exact = /sources\s+rename\s+<old>\s+<new>/i.test(out)
+      || /sources\s+rename\s+OLD\s+NEW/.test(out)
+      || /sources\s+rename\s+<oldId>\s+<newId>/i.test(out);
+    _gbrainSupportsRenameCache = exact && r.status === 0;
+  } catch {
+    _gbrainSupportsRenameCache = false;
+  }
+  return _gbrainSupportsRenameCache;
+}
+
+/**
+ * Look up a source's `local_path` from `gbrain sources list --json`.
+ * Returns null when the source is absent or the listing fails.
+ *
+ * `env` is the environment passed to the spawned `gbrain` process; defaults
+ * to `process.env`. Tests inject a PATH that points at a gbrain shim so the
+ * helper can be exercised without a real gbrain CLI.
+ */
+export function sourceLocalPath(sourceId: string, env?: NodeJS.ProcessEnv): string | null {
+  try {
+    const r = spawnSync("gbrain", ["sources", "list", "--json"], {
+      encoding: "utf-8",
+      timeout: 30_000,
+      stdio: ["ignore", "pipe", "pipe"],
+      env: env || process.env,
+    });
+    if (r.status !== 0) return null;
+    const list = JSON.parse(r.stdout || "[]") as Array<{ id: string; local_path?: string }>;
+    const found = list.find((s) => s.id === sourceId);
+    return found?.local_path ?? null;
+  } catch {
+    return null;
+  }
+}
+
+/** Result of `planHostnameFoldMigration` — informs `runCodeImport` of next steps. */
+export type HostnameFoldMigration =
+  | { kind: "none"; reason: "ids-match" | "no-legacy-source" }
+  | { kind: "skipped-path-drift"; oldId: string; oldPath: string; currentPath: string }
+  | { kind: "renamed"; oldId: string; newId: string }
+  | { kind: "pending-cleanup"; oldId: string };
+
+/**
+ * Decide how to migrate from the pre-#1468 path-only-hash source id to the
+ * new hostname-fold id.
+ *
+ * Order:
+ *   1. If old == new → no-op.
+ *   2. Look up old source's local_path. Absent → no legacy source to migrate.
+ *   3. local_path != currentRoot → user moved the repo or two machines share a
+ *      hash slot. Skip migration; let the user clean up manually. We will NOT
+ *      rename or remove anything; the new source is registered alongside.
+ *   4. Otherwise: feature-check `gbrain sources rename`. If supported and the
+ *      rename call exits 0 → renamed, pages preserved.
+ *   5. Else: pending-cleanup. Caller registers + syncs new source first; only
+ *      after sync succeeds with a non-zero page count does it remove the old.
+ *      This avoids a data-loss window where the old source is gone before the
+ *      new one is verifiably populated.
+ */
+export function planHostnameFoldMigration(
+  currentRoot: string,
+  newSourceId: string,
+  legacyPathHashId: string,
+  env?: NodeJS.ProcessEnv,
+): HostnameFoldMigration {
+  if (legacyPathHashId === newSourceId) {
+    return { kind: "none", reason: "ids-match" };
+  }
+  const oldPath = sourceLocalPath(legacyPathHashId, env);
+  if (oldPath === null) {
+    return { kind: "none", reason: "no-legacy-source" };
+  }
+  if (oldPath !== currentRoot) {
+    return {
+      kind: "skipped-path-drift",
+      oldId: legacyPathHashId,
+      oldPath,
+      currentPath: currentRoot,
+    };
+  }
+  if (gbrainSupportsSourcesRename(env)) {
+    const r = spawnSync("gbrain", ["sources", "rename", legacyPathHashId, newSourceId], {
+      encoding: "utf-8",
+      timeout: 30_000,
+      stdio: ["ignore", "pipe", "pipe"],
+      env: env || process.env,
+    });
+    if (r.status === 0) {
+      return { kind: "renamed", oldId: legacyPathHashId, newId: newSourceId };
+    }
+    // Rename failed at runtime — fall through to cleanup path.
+  }
+  return { kind: "pending-cleanup", oldId: legacyPathHashId };
+}
+
+/**
+ * Remove an orphaned source. Called only after new-source sync verifies pages
+ * exist, so the old source is provably redundant before deletion.
+ *
+ * Flag note: existing call sites used `--confirm-destructive` here and
+ * `--yes` in `lib/gbrain-sources.ts` — gbrain 0.35.0.0 accepts neither
+ * deterministically (the subcommand surface help is generic). We pass
+ * `--confirm-destructive` to match the existing call site convention; the
+ * flag-helper centralization in commit 4 (lib/gbrain-exec.ts) will resolve
+ * the inconsistency across the codebase.
+ */
+export function removeOrphanedSource(oldId: string): boolean {
+  const r = spawnSync("gbrain", ["sources", "remove", oldId, "--confirm-destructive"], {
+    encoding: "utf-8",
+    timeout: 30_000,
+    stdio: ["ignore", "pipe", "pipe"],
+  });
+  return r.status === 0;
+}
+
 /**
  * Build a gbrain-valid source id (1-32 lowercase alnum + interior hyphens). Sanitizes
  * `raw`, prefixes with `prefix`, and falls back to a hashed-tail form when total length
@@ -365,7 +536,7 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
     return skipStageForLocalStatus("code", localStatus, t0);
   }
 
-  // Step 0: Best-effort cleanup of pre-pathhash legacy source.
+  // Step 0a: Best-effort cleanup of pre-pathhash legacy source (v1.x form).
   // Earlier /sync-gbrain versions registered `gstack-code-<slug>` (no path
   // suffix). On a multi-worktree repo, those collapsed onto a single id
   // with last-sync-wins. Federated search would return stale duplicate
@@ -385,6 +556,24 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
     if (rm.status === 0) legacyRemoved = true;
   }
 
+  // Step 0b: Hostname-fold migration (#1414).
+  // Before #1468 the source id hashed only the absolute repo path. After the
+  // hostname fold, every existing user has a legacy id that no longer matches
+  // what deriveCodeSourceId produces. Try rename-in-place first (preserves
+  // pages); fall back to register-new → sync-OK → remove-old. Path-drift
+  // (user moved the repo, etc.) skips migration with a warning.
+  const pathOnlyHashLegacyId = derivePathOnlyHashLegacyId(root);
+  const migration = planHostnameFoldMigration(root, sourceId, pathOnlyHashLegacyId);
+  if (migration.kind === "skipped-path-drift" && !args.quiet) {
+    console.error(
+      `[sync:code] hostname-fold migration skipped: legacy source ${migration.oldId} `
+      + `points at ${migration.oldPath}, current repo is ${migration.currentPath}. `
+      + `Clean up manually with: gbrain sources remove ${migration.oldId} --confirm-destructive`,
+    );
+  } else if (migration.kind === "renamed" && !args.quiet) {
+    console.error(`[sync:code] hostname-fold migration: renamed ${migration.oldId} → ${migration.newId} (pages preserved)`);
+  }
+
   // Step 1: Ensure source registered (idempotent). Single source of truth in lib —
   // no synchronous duplicate here (per /codex review #12).
   let registered = false;
@@ -439,7 +628,26 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
     stdio: ["ignore", "pipe", "pipe"],
   });
   const pageCount = sourcePageCount(sourceId);
-  const legacyNote = legacyRemoved ? `, removed legacy ${legacyId}` : "";
+
+  // Step 4: Deferred hostname-fold cleanup.
+  // Only remove the pre-#1468 path-only-hash source NOW that the new source
+  // has registered + synced + has pages. Removing before sync would create a
+  // data-loss window if sync failed; removing without a page-count check would
+  // wipe pages when sync silently no-op'd. This is the codex-review-flagged
+  // safety: register → sync → verify → THEN delete.
+  let hostnameLegacyRemoved = false;
+  if (migration.kind === "pending-cleanup" && pageCount !== null && pageCount > 0) {
+    hostnameLegacyRemoved = removeOrphanedSource(migration.oldId);
+    if (hostnameLegacyRemoved && !args.quiet) {
+      console.error(`[sync:code] hostname-fold migration: removed legacy ${migration.oldId} after new source sync verified (page_count=${pageCount})`);
+    }
+  }
+
+  const legacyParts: string[] = [];
+  if (legacyRemoved) legacyParts.push(`removed legacy ${legacyId}`);
+  if (migration.kind === "renamed") legacyParts.push(`renamed ${migration.oldId}→${migration.newId}`);
+  if (hostnameLegacyRemoved) legacyParts.push(`removed pre-hostname-fold ${migration.kind === "pending-cleanup" ? migration.oldId : ""}`);
+  const legacyNote = legacyParts.length > 0 ? `, ${legacyParts.join(", ")}` : "";
   const baseSummary = `${registered ? "registered + " : ""}synced ${sourceId} (page_count=${pageCount ?? "unknown"}${legacyNote})`;
 
   if (attach.status !== 0) {
@@ -675,8 +883,10 @@ async function main(): Promise<void> {
   process.exit(exitCode);
 }
 
-main().catch((err) => {
-  console.error(`gstack-gbrain-sync fatal: ${err instanceof Error ? err.message : String(err)}`);
-  releaseLock();
-  process.exit(1);
-});
+if (import.meta.main) {
+  main().catch((err) => {
+    console.error(`gstack-gbrain-sync fatal: ${err instanceof Error ? err.message : String(err)}`);
+    releaseLock();
+    process.exit(1);
+  });
+}
diff --git a/test/gstack-gbrain-sync.test.ts b/test/gstack-gbrain-sync.test.ts
index 528d6deed..74b0e2e8f 100644
--- a/test/gstack-gbrain-sync.test.ts
+++ b/test/gstack-gbrain-sync.test.ts
@@ -7,12 +7,19 @@
  * preview + state file lifecycle + flag composition.
  */
 
-import { describe, it, expect } from "bun:test";
-import { mkdtempSync, writeFileSync, readFileSync, existsSync, rmSync, mkdirSync } from "fs";
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+import { mkdtempSync, writeFileSync, readFileSync, existsSync, rmSync, mkdirSync, chmodSync } from "fs";
 import { tmpdir } from "os";
 import { join } from "path";
 import { spawnSync } from "child_process";
 
+import {
+  derivePathOnlyHashLegacyId,
+  planHostnameFoldMigration,
+  sourceLocalPath,
+  _resetGbrainSupportsRenameCache,
+} from "../bin/gstack-gbrain-sync";
+
 const SCRIPT = join(import.meta.dir, "..", "bin", "gstack-gbrain-sync.ts");
 
 function makeTestHome(): string {
@@ -215,6 +222,62 @@ describe("gstack-gbrain-sync CLI", () => {
     rmSync(home, { recursive: true, force: true });
   });
 
+  it("derives distinct source ids for the same absolute path on different hosts", () => {
+    // Issue #1414: two machines with identical home-dir layouts (chezmoi-managed
+    // dotfiles, ansible-provisioned VMs) collide on the same source id when
+    // federated against a shared gbrain DB, because the pre-fix `pathHash` was
+    // sha1(absolute path) only — host-agnostic. Folding hostname into the hash
+    // key keeps them distinct. `GSTACK_HOSTNAME` env var is the test-only knob;
+    // production uses `os.hostname()`.
+    const home = makeTestHome();
+    const gstackHome = join(home, ".gstack");
+    mkdirSync(gstackHome, { recursive: true });
+    const repo = mkdtempSync(join(tmpdir(), "gstack-host-collide-"));
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    spawnSync("git", ["remote", "add", "origin", "https://github.com/example/multihost.git"], { cwd: repo });
+
+    // Dry-run still gates the code stage on `command -v gbrain`. Drop a no-op
+    // shim on PATH so the stage runs (we only assert the preview line, never
+    // invoke gbrain itself).
+    const bindir = mkdtempSync(join(tmpdir(), "gstack-host-collide-bin-"));
+    const shim = join(bindir, "gbrain");
+    writeFileSync(shim, "#!/bin/sh\nexit 0\n");
+    chmodSync(shim, 0o755);
+    const PATH = `${bindir}:${process.env.PATH || ""}`;
+
+    const runAs = (host: string) =>
+      spawnSync("bun", [SCRIPT, "--dry-run", "--code-only", "--quiet"], {
+        encoding: "utf-8",
+        timeout: 60000,
+        cwd: repo,
+        env: { ...process.env, HOME: home, GSTACK_HOME: gstackHome, GSTACK_HOSTNAME: host, PATH },
+      });
+
+    const a = runAs("machine-a");
+    const b = runAs("machine-b");
+    expect(a.status).toBe(0);
+    expect(b.status).toBe(0);
+    const idA = (a.stdout || "").match(/gbrain sources add (\S+)/)?.[1];
+    const idB = (b.stdout || "").match(/gbrain sources add (\S+)/)?.[1];
+    expect(idA).toBeTruthy();
+    expect(idB).toBeTruthy();
+    expect(idA).not.toBe(idB);
+    // Both still gbrain-valid.
+    const VALID_ID = /^[a-z0-9](?:[a-z0-9-]{0,30}[a-z0-9])?$/;
+    expect(idA!).toMatch(VALID_ID);
+    expect(idB!).toMatch(VALID_ID);
+
+    // Same host + same path stays stable across invocations.
+    const a2 = runAs("machine-a");
+    expect(a2.status).toBe(0);
+    const idA2 = (a2.stdout || "").match(/gbrain sources add (\S+)/)?.[1];
+    expect(idA2).toBe(idA);
+
+    rmSync(repo, { recursive: true, force: true });
+    rmSync(home, { recursive: true, force: true });
+    rmSync(bindir, { recursive: true, force: true });
+  });
+
   it("dry-run does NOT acquire the lock file (lock is for write paths only)", () => {
     const home = makeTestHome();
     const gstackHome = join(home, ".gstack");
@@ -476,3 +539,227 @@ describe("gstack-gbrain-sync CLI", () => {
     rmSync(home, { recursive: true, force: true });
   });
 });
+
+// ──────────────────────────────────────────────────────────────────────────
+// Hostname-fold migration (v1.40.0.0)
+//
+// Tests for `derivePathOnlyHashLegacyId` and `planHostnameFoldMigration`,
+// which together let an existing user's pre-#1468 path-only-hash source
+// transition to the new hostname-folded id without orphaning pages or
+// creating a data-loss window. See bin/gstack-gbrain-sync.ts and the
+// gbrain-sync-hardening plan.
+// ──────────────────────────────────────────────────────────────────────────
+
+/**
+ * Build a gbrain shim that responds to specific subcommands with canned
+ * output, then return PATH-prepend value. Lets us run helpers in-process
+ * (which spawn `gbrain` from PATH) without a real gbrain CLI.
+ */
+function makeShim(bindir: string, responses: Record<string, { stdout?: string; stderr?: string; exit?: number }>): string {
+  const shim = join(bindir, "gbrain");
+  const cases = Object.entries(responses).map(([key, r]) => {
+    const exit = r.exit ?? 0;
+    const stdout = (r.stdout || "").replace(/'/g, "'\\''");
+    const stderr = (r.stderr || "").replace(/'/g, "'\\''");
+    // Patterns with spaces MUST be double-quoted in sh case statements,
+    // otherwise the shell parses the second word as the start of the next
+    // pattern and errors out.
+    return `  "${key}") printf '%s' '${stdout}'; printf '%s' '${stderr}' >&2; exit ${exit} ;;`;
+  }).join("\n");
+  // Match on the full argument string, joined with literal spaces.
+  const script = `#!/bin/sh\nARGS="$*"\ncase "$ARGS" in\n${cases}\n  *) echo "shim: no match for [$ARGS]" >&2; exit 1 ;;\nesac\n`;
+  writeFileSync(shim, script);
+  chmodSync(shim, 0o755);
+  return shim;
+}
+
+describe("derivePathOnlyHashLegacyId", () => {
+  it("returns the pre-#1468 form (path-only sha1, no hostname)", () => {
+    // Pure function — no subprocess. The same repoPath must yield the same
+    // legacy id regardless of $GSTACK_HOSTNAME, because the pre-#1468 hash
+    // didn't include hostname.
+    const repo = mkdtempSync(join(tmpdir(), "gstack-legacy-id-"));
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    spawnSync("git", ["remote", "add", "origin", "https://github.com/example/legacy-test.git"], { cwd: repo });
+
+    const cwd = process.cwd();
+    try {
+      process.chdir(repo);
+      const a = derivePathOnlyHashLegacyId(repo);
+      process.env.GSTACK_HOSTNAME = "machine-a";
+      const b = derivePathOnlyHashLegacyId(repo);
+      process.env.GSTACK_HOSTNAME = "machine-b";
+      const c = derivePathOnlyHashLegacyId(repo);
+      expect(a).toBe(b);
+      expect(b).toBe(c);
+      expect(a.startsWith("gstack-code-")).toBe(true);
+      expect(a.length).toBeLessThanOrEqual(32);
+    } finally {
+      delete process.env.GSTACK_HOSTNAME;
+      process.chdir(cwd);
+      rmSync(repo, { recursive: true, force: true });
+    }
+  });
+
+  it("produces a different id than the new hostname-folded form", () => {
+    // The whole point of the migration: the path-only-hash legacy id and the
+    // host-fold id must differ for any non-empty hostname, so the migration
+    // can detect + clean up the orphan.
+    const repo = mkdtempSync(join(tmpdir(), "gstack-legacy-id-distinct-"));
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    spawnSync("git", ["remote", "add", "origin", "https://github.com/example/distinct.git"], { cwd: repo });
+
+    const cwd = process.cwd();
+    try {
+      process.chdir(repo);
+      process.env.GSTACK_HOSTNAME = "machine-x";
+      const legacy = derivePathOnlyHashLegacyId(repo);
+      // Drive the new id through the CLI so we use the same code path users hit.
+      const home = makeTestHome();
+      const gstackHome = join(home, ".gstack");
+      mkdirSync(gstackHome, { recursive: true });
+      const bindir = mkdtempSync(join(tmpdir(), "gstack-legacy-id-distinct-bin-"));
+      makeShim(bindir, { "--help": { stdout: "gbrain\n" } });
+      const r = spawnSync("bun", [SCRIPT, "--dry-run", "--code-only", "--quiet"], {
+        encoding: "utf-8",
+        timeout: 60000,
+        cwd: repo,
+        env: { ...process.env, HOME: home, GSTACK_HOME: gstackHome, GSTACK_HOSTNAME: "machine-x", PATH: `${bindir}:${process.env.PATH || ""}` },
+      });
+      const newId = (r.stdout || "").match(/gbrain sources add (\S+)/)?.[1];
+      expect(newId).toBeTruthy();
+      expect(newId).not.toBe(legacy);
+      rmSync(home, { recursive: true, force: true });
+      rmSync(bindir, { recursive: true, force: true });
+    } finally {
+      delete process.env.GSTACK_HOSTNAME;
+      process.chdir(cwd);
+      rmSync(repo, { recursive: true, force: true });
+    }
+  });
+});
+
+/**
+ * Build an env dict that prepends `bindir` to PATH. Bun's spawnSync does NOT
+ * pick up runtime mutations of `process.env.PATH` — the env must be passed
+ * explicitly to each spawn for the override to take effect.
+ */
+function envWithBindir(bindir: string): NodeJS.ProcessEnv {
+  return { ...process.env, PATH: `${bindir}:${process.env.PATH || ""}` };
+}
+
+describe("planHostnameFoldMigration", () => {
+  let bindir: string;
+
+  beforeEach(() => {
+    bindir = mkdtempSync(join(tmpdir(), "gstack-mig-plan-bin-"));
+    _resetGbrainSupportsRenameCache();
+  });
+  afterEach(() => {
+    rmSync(bindir, { recursive: true, force: true });
+    _resetGbrainSupportsRenameCache();
+  });
+
+  it("returns ids-match when legacy == new (degenerate case)", () => {
+    const result = planHostnameFoldMigration("/repo/path", "gstack-code-same-abc12345", "gstack-code-same-abc12345");
+    expect(result).toEqual({ kind: "none", reason: "ids-match" });
+  });
+
+  it("returns no-legacy-source when sources list does not include the legacy id", () => {
+    makeShim(bindir, {
+      "sources list --json": { stdout: "[]" },
+    });
+    const result = planHostnameFoldMigration("/repo/path", "new-id", "legacy-id", envWithBindir(bindir));
+    expect(result).toEqual({ kind: "none", reason: "no-legacy-source" });
+  });
+
+  it("returns skipped-path-drift when old source local_path differs from current repo root", () => {
+    makeShim(bindir, {
+      "sources list --json": {
+        stdout: JSON.stringify([{ id: "legacy-id", local_path: "/some/other/repo" }]),
+      },
+    });
+    const result = planHostnameFoldMigration("/repo/here", "new-id", "legacy-id", envWithBindir(bindir));
+    expect(result.kind).toBe("skipped-path-drift");
+    if (result.kind === "skipped-path-drift") {
+      expect(result.oldId).toBe("legacy-id");
+      expect(result.oldPath).toBe("/some/other/repo");
+      expect(result.currentPath).toBe("/repo/here");
+    }
+  });
+
+  it("returns renamed when rename is supported and exits 0", () => {
+    makeShim(bindir, {
+      "sources list --json": {
+        stdout: JSON.stringify([{ id: "legacy-id", local_path: "/repo/here" }]),
+      },
+      "sources rename --help": {
+        stdout: "Usage: gbrain sources rename <old> <new>\n",
+      },
+      "sources rename legacy-id new-id": { exit: 0 },
+    });
+    const result = planHostnameFoldMigration("/repo/here", "new-id", "legacy-id", envWithBindir(bindir));
+    expect(result).toEqual({ kind: "renamed", oldId: "legacy-id", newId: "new-id" });
+  });
+
+  it("returns pending-cleanup when rename is unsupported (current gbrain 0.35.0.0)", () => {
+    makeShim(bindir, {
+      "sources list --json": {
+        stdout: JSON.stringify([{ id: "legacy-id", local_path: "/repo/here" }]),
+      },
+      // No `sources rename --help` match → shim falls into the catch-all and exits 1.
+    });
+    const result = planHostnameFoldMigration("/repo/here", "new-id", "legacy-id", envWithBindir(bindir));
+    expect(result).toEqual({ kind: "pending-cleanup", oldId: "legacy-id" });
+  });
+
+  it("returns pending-cleanup when rename is supported but the rename call itself fails", () => {
+    makeShim(bindir, {
+      "sources list --json": {
+        stdout: JSON.stringify([{ id: "legacy-id", local_path: "/repo/here" }]),
+      },
+      "sources rename --help": {
+        stdout: "Usage: gbrain sources rename <old> <new>\n",
+      },
+      "sources rename legacy-id new-id": { exit: 1, stderr: "rename failed: db locked" },
+    });
+    const result = planHostnameFoldMigration("/repo/here", "new-id", "legacy-id", envWithBindir(bindir));
+    expect(result).toEqual({ kind: "pending-cleanup", oldId: "legacy-id" });
+  });
+});
+
+describe("sourceLocalPath", () => {
+  let bindir: string;
+  beforeEach(() => {
+    bindir = mkdtempSync(join(tmpdir(), "gstack-source-lp-bin-"));
+  });
+  afterEach(() => {
+    rmSync(bindir, { recursive: true, force: true });
+  });
+
+  it("returns local_path when the source exists", () => {
+    makeShim(bindir, {
+      "sources list --json": {
+        stdout: JSON.stringify([
+          { id: "other-source", local_path: "/x" },
+          { id: "target-id", local_path: "/repo/match" },
+        ]),
+      },
+    });
+    expect(sourceLocalPath("target-id", envWithBindir(bindir))).toBe("/repo/match");
+  });
+
+  it("returns null when the source is missing", () => {
+    makeShim(bindir, {
+      "sources list --json": { stdout: "[]" },
+    });
+    expect(sourceLocalPath("missing-id", envWithBindir(bindir))).toBeNull();
+  });
+
+  it("returns null when gbrain exits non-zero or returns malformed JSON", () => {
+    makeShim(bindir, {
+      "sources list --json": { exit: 2, stderr: "db unreachable" },
+    });
+    expect(sourceLocalPath("any-id", envWithBindir(bindir))).toBeNull();
+  });
+});