diff --git a/bin/gstack-gbrain-sync.ts b/bin/gstack-gbrain-sync.ts
index 2624dd6ba..e98629718 100644
--- a/bin/gstack-gbrain-sync.ts
+++ b/bin/gstack-gbrain-sync.ts
@@ -29,7 +29,7 @@
  * than building a gstack-side daemon.
  */
 
-import { existsSync, statSync, mkdirSync, writeFileSync, readFileSync, unlinkSync, renameSync } from "fs";
+import { existsSync, statSync, mkdirSync, writeFileSync, readFileSync, unlinkSync, renameSync, realpathSync } from "fs";
 import { join, dirname } from "path";
 import { execSync, spawnSync } from "child_process";
 import { homedir, hostname } from "os";
@@ -368,6 +368,42 @@ function deriveCodeSourceId(repoPath: string): string {
   return constrainSourceId("gstack-code", `${base}-${hostPathHash}`);
 }
 
+/**
+ * Reuse an explicit repo pin when it names a registered source for this exact
+ * checkout. The path check prevents a stale or copied dotfile from redirecting
+ * a code sync into another repo's source.
+ */
+function readPinnedSourceId(repoPath: string): string | null {
+  const pinPath = join(repoPath, ".gbrain-source");
+  if (!existsSync(pinPath)) return null;
+
+  try {
+    const sourceId = readFileSync(pinPath, "utf-8").trim();
+    return /^[a-z0-9](?:[a-z0-9-]{0,30}[a-z0-9])?$/.test(sourceId) ? sourceId : null;
+  } catch {
+    // A pin is advisory. A permission race or a directory at this path must
+    // not turn a sync preview into an unexpected crash.
+    return null;
+  }
+}
+
+export function existingPinnedSourceId(repoPath: string, env?: NodeJS.ProcessEnv): string | null {
+  const sourceId = readPinnedSourceId(repoPath);
+  if (!sourceId) return null;
+
+  const registeredPath = sourceLocalPath(sourceId, env);
+  if (!registeredPath) return null;
+  try {
+    return realpathSync(registeredPath) === realpathSync(repoPath) ? sourceId : null;
+  } catch {
+    return null;
+  }
+}
+
+function resolveCodeSourceId(repoPath: string, env?: NodeJS.ProcessEnv): string {
+  return existingPinnedSourceId(repoPath, env) ?? deriveCodeSourceId(repoPath);
+}
+
 /**
  * Pre-pathhash source id, kept for orphan detection only.
  *
@@ -820,7 +856,13 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
     return { name: "code", ran: false, ok: true, duration_ms: 0, summary: "skipped (not in git repo)" };
   }
 
-  const sourceId = deriveCodeSourceId(root);
+  // A preview must not spawn gbrain. Trust a syntactically-valid local pin
+  // there; a real run confirms its registered path before using it.
+  const gbrainEnv = args.mode === "dry-run" ? undefined : buildGbrainEnv({ announce: !args.quiet });
+  const pinnedSourceId = args.mode === "dry-run"
+    ? readPinnedSourceId(root)
+    : existingPinnedSourceId(root, gbrainEnv);
+  const sourceId = pinnedSourceId ?? deriveCodeSourceId(root);
 
   // Per-repo trust tier — checked BEFORE the dry-run branch so previews report
   // the refusal honestly instead of claiming they would sync.
@@ -861,7 +903,9 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
       ran: false,
       ok: true,
       duration_ms: 0,
-      summary: `would: gbrain sources add ${sourceId} --path ${root} --federated; gbrain sync --strategy code --source ${sourceId}; gbrain sources attach ${sourceId}`,
+      summary: pinnedSourceId
+        ? `would: gbrain sync --strategy code --source ${sourceId}; gbrain sources attach ${sourceId}`
+        : `would: gbrain sources add ${sourceId} --path ${root} --federated; gbrain sync --strategy code --source ${sourceId}; gbrain sources attach ${sourceId}`,
       detail: { source_id: sourceId, source_path: root, status: "skipped" },
     };
   }
@@ -889,10 +933,9 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
   // gbrainEnv seeds DATABASE_URL from gbrain's config so this stage works
   // inside Next.js / Prisma / Rails projects with their own .env.local
   // (codex review #7 — bug fix is wider than #1508 as filed).
-  const gbrainEnv = buildGbrainEnv({ announce: !args.quiet });
   const legacyId = deriveLegacyCodeSourceId(root);
   let legacyRemoved = false;
-  if (legacyId !== sourceId) {
+  if (!pinnedSourceId && legacyId !== sourceId) {
     // #1734: route through the data-loss guards (autopilot + source-safety).
     const rm = safeSourcesRemove(legacyId, gbrainEnv);
     if (rm.skipped && !args.quiet) {
@@ -908,7 +951,9 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
   // 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, gbrainEnv);
+  const migration = pinnedSourceId
+    ? { kind: "none", reason: "no-legacy-source" } as const
+    : planHostnameFoldMigration(root, sourceId, pathOnlyHashLegacyId, gbrainEnv);
   if (migration.kind === "skipped-path-drift" && !args.quiet) {
     console.error(
       `[sync:code] hostname-fold migration skipped: legacy source ${migration.oldId} `
@@ -919,21 +964,24 @@ async function runCodeImport(args: CliArgs): Promise<StageResult> {
     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).
+  // Step 1: Ensure generated sources are registered. A confirmed explicit pin
+  // belongs to the user: its realpath was checked above, so never remove/add it
+  // merely because the registered spelling differs (e.g. a symlinked checkout).
   let registered = false;
-  try {
-    const result = await ensureSourceRegistered(sourceId, root, { federated: true, env: gbrainEnv });
-    registered = result.changed;
-  } catch (err) {
-    return {
-      name: "code",
-      ran: true,
-      ok: false,
-      duration_ms: Date.now() - t0,
-      summary: `source registration failed: ${(err as Error).message}`,
-      detail: { source_id: sourceId, source_path: root, status: "failed" },
-    };
+  if (!pinnedSourceId) {
+    try {
+      const result = await ensureSourceRegistered(sourceId, root, { federated: true, env: gbrainEnv });
+      registered = result.changed;
+    } catch (err) {
+      return {
+        name: "code",
+        ran: true,
+        ok: false,
+        duration_ms: Date.now() - t0,
+        summary: `source registration failed: ${(err as Error).message}`,
+        detail: { source_id: sourceId, source_path: root, status: "failed" },
+      };
+    }
   }
 
   // Step 2: Always run the page-creating file walk first, then (for --full)
@@ -1336,7 +1384,7 @@ export async function runDream(args: CliArgs): Promise<StageResult> {
 
   if (args.mode === "dry-run") {
     const root = repoRoot();
-    const sourceId = root ? deriveCodeSourceId(root) : null;
+    const sourceId = root ? readPinnedSourceId(root) ?? deriveCodeSourceId(root) : null;
     return {
       name: "dream",
       ran: false,
@@ -1348,6 +1396,7 @@ export async function runDream(args: CliArgs): Promise<StageResult> {
     };
   }
 
+  const gbrainEnv = buildGbrainEnv({ announce: !args.quiet });
   const localStatus = localEngineStatus({ noCache: false });
   if (localStatus === "timeout") {
     warnProbeTimeout("dream"); // #1964: slow-but-healthy — proceed
@@ -1383,7 +1432,7 @@ export async function runDream(args: CliArgs): Promise<StageResult> {
     // code-callers/code-callees for this worktree. Falls back to plain `dream`
     // only when we can't derive the source id (not in a git repo).
     const root = repoRoot();
-    const sourceId = root ? deriveCodeSourceId(root) : null;
+    const sourceId = root ? resolveCodeSourceId(root, gbrainEnv) : null;
     const dreamArgs = sourceId ? ["dream", "--source", sourceId] : ["dream"];
 
     // spawnGbrain seeds DATABASE_URL from gbrain's config via buildGbrainEnv.
@@ -1675,7 +1724,8 @@ async function main(): Promise<void> {
     let cycle: CycleStatus | null = null;
     if (!args.dream && args.mode === "full" && !args.noDream && !args.noCode) {
       const root = repoRoot();
-      cycle = root ? cycleCompleted(deriveCodeSourceId(root), process.env) : "unknown";
+      const gbrainEnv = buildGbrainEnv({ announce: !args.quiet });
+      cycle = root ? cycleCompleted(resolveCodeSourceId(root, gbrainEnv), gbrainEnv) : "unknown";
     }
     if (shouldRunDream(args, cycle)) {
       dreamStage = await runDream(args);
diff --git a/test/gstack-gbrain-sync.test.ts b/test/gstack-gbrain-sync.test.ts
index 19a9bac4e..9948da51c 100644
--- a/test/gstack-gbrain-sync.test.ts
+++ b/test/gstack-gbrain-sync.test.ts
@@ -8,7 +8,7 @@
  */
 
 import { describe, it, expect, beforeEach, afterEach } from "bun:test";
-import { mkdtempSync, writeFileSync, readFileSync, existsSync, rmSync, mkdirSync, chmodSync } from "fs";
+import { mkdtempSync, writeFileSync, readFileSync, existsSync, rmSync, mkdirSync, chmodSync, symlinkSync } from "fs";
 import { tmpdir } from "os";
 import { join } from "path";
 import { spawnSync } from "child_process";
@@ -62,6 +62,14 @@ describe("gstack-gbrain-sync CLI", () => {
     expect(source).toContain("localEngineStatus");
   });
 
+  it("uses GBrain's config environment when resolving dream sources", () => {
+    const source = readFileSync(SCRIPT, "utf-8");
+
+    expect(source).not.toContain("resolveCodeSourceId(root, process.env)");
+    expect(source).toContain("resolveCodeSourceId(root, gbrainEnv)");
+    expect(source).toContain("cycleCompleted(resolveCodeSourceId(root, gbrainEnv), gbrainEnv)");
+  });
+
   it("--dry-run with --code-only reports the code import preview only", () => {
     const home = makeTestHome();
     const gstackHome = join(home, ".gstack");
@@ -122,6 +130,139 @@ describe("gstack-gbrain-sync CLI", () => {
     rmSync(home, { recursive: true, force: true });
   });
 
+  it("uses a local .gbrain-source in dry-run without spawning gbrain", () => {
+    const home = makeTestHome();
+    const gstackHome = join(home, ".gstack");
+    const bindir = mkdtempSync(join(tmpdir(), "gstack-pinned-source-bin-"));
+    const repo = mkdtempSync(join(tmpdir(), "gstack-pinned-source-repo-"));
+    const commandLog = join(home, "gbrain-commands.log");
+    mkdirSync(gstackHome, { recursive: true });
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    writeFileSync(join(repo, ".gbrain-source"), "client-acme-app\n");
+    writeFileSync(join(bindir, "gbrain"), `#!/bin/sh
+printf '%s\\n' "$*" >> "$GSTACK_TEST_GBRAIN_LOG"
+exit 99
+`);
+    chmodSync(join(bindir, "gbrain"), 0o755);
+
+    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_TEST_GBRAIN_LOG: commandLog,
+        PATH: `${bindir}:${process.env.PATH || ""}`,
+      },
+    });
+
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("gbrain sync --strategy code --source client-acme-app");
+    expect(r.stdout).not.toContain("gbrain sources add");
+    expect(r.stdout).not.toContain("--federated");
+    expect(existsSync(commandLog)).toBe(false);
+    rmSync(repo, { recursive: true, force: true });
+    rmSync(bindir, { recursive: true, force: true });
+    rmSync(home, { recursive: true, force: true });
+  });
+
+  it("keeps a symlink-equivalent pinned source registered as-is", () => {
+    const home = makeTestHome();
+    const gstackHome = join(home, ".gstack");
+    const repo = mkdtempSync(join(tmpdir(), "gstack-pinned-source-repo-"));
+    const linkDir = mkdtempSync(join(tmpdir(), "gstack-pinned-source-link-"));
+    const link = join(linkDir, "repo");
+    const bindir = mkdtempSync(join(tmpdir(), "gstack-pinned-source-bin-"));
+    const commandLog = join(home, "gbrain-commands.log");
+    mkdirSync(gstackHome, { recursive: true });
+    mkdirSync(join(home, ".gbrain"), { recursive: true });
+    writeFileSync(join(home, ".gbrain", "config.json"), JSON.stringify({ engine: "pglite", database_url: "pglite:///test" }));
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    writeFileSync(join(repo, ".gbrain-source"), "client-acme-app\n");
+    symlinkSync(repo, link, "dir");
+    writeFileSync(join(bindir, "gbrain"), `#!/bin/sh
+printf '%s\\n' "$*" >> "$GSTACK_TEST_GBRAIN_LOG"
+case "$*" in
+  --version) echo 'gbrain 0.42.0.0' ;;
+  "sources list --json") echo '{"sources":[{"id":"client-acme-app","local_path":"${link}","page_count":1}]}' ;;
+  "sync --strategy code --source client-acme-app"|"sources attach client-acme-app") ;;
+  *) echo "unexpected gbrain command: $*" >&2; exit 1 ;;
+esac
+`);
+    chmodSync(join(bindir, "gbrain"), 0o755);
+
+    const r = spawnSync("bun", [SCRIPT, "--code-only", "--quiet"], {
+      encoding: "utf-8",
+      timeout: 60000,
+      cwd: link,
+      env: {
+        ...process.env,
+        HOME: home,
+        GSTACK_HOME: gstackHome,
+        GSTACK_TEST_GBRAIN_LOG: commandLog,
+        PATH: `${bindir}:${process.env.PATH || ""}`,
+      },
+    });
+
+    const commands = readFileSync(commandLog, "utf-8");
+    expect(r.status).toBe(0);
+    expect(commands).toContain("sync --strategy code --source client-acme-app");
+    expect(commands).toContain("sources attach client-acme-app");
+    expect(commands).not.toMatch(/^sources (add|remove) /m);
+    rmSync(repo, { recursive: true, force: true });
+    rmSync(linkDir, { recursive: true, force: true });
+    rmSync(bindir, { recursive: true, force: true });
+    rmSync(home, { recursive: true, force: true });
+  });
+
+  it("uses a local pin for a dry-run dream without spawning gbrain", () => {
+    const home = makeTestHome();
+    const gstackHome = join(home, ".gstack");
+    const bindir = mkdtempSync(join(tmpdir(), "gstack-pinned-dream-bin-"));
+    const repo = mkdtempSync(join(tmpdir(), "gstack-pinned-dream-repo-"));
+    mkdirSync(gstackHome, { recursive: true });
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    writeFileSync(join(repo, ".gbrain-source"), "client-acme-app\n");
+    writeFileSync(join(bindir, "gbrain"), "#!/bin/sh\nexit 99\n");
+    chmodSync(join(bindir, "gbrain"), 0o755);
+
+    const r = spawnSync("bun", [SCRIPT, "--dry-run", "--dream", "--no-code", "--no-memory", "--no-brain-sync", "--quiet"], {
+      encoding: "utf-8",
+      timeout: 60000,
+      cwd: repo,
+      env: { ...process.env, HOME: home, GSTACK_HOME: gstackHome, PATH: `${bindir}:${process.env.PATH || ""}` },
+    });
+
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("gbrain dream --source client-acme-app");
+    rmSync(repo, { recursive: true, force: true });
+    rmSync(bindir, { recursive: true, force: true });
+    rmSync(home, { recursive: true, force: true });
+  });
+
+  it("falls back to a derived source when .gbrain-source cannot be read", () => {
+    const home = makeTestHome();
+    const gstackHome = join(home, ".gstack");
+    const repo = mkdtempSync(join(tmpdir(), "gstack-unreadable-pin-repo-"));
+    mkdirSync(gstackHome, { recursive: true });
+    spawnSync("git", ["init", "--quiet", "-b", "main"], { cwd: repo });
+    mkdirSync(join(repo, ".gbrain-source"));
+
+    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 },
+    });
+
+    expect(r.status).toBe(0);
+    expect(r.stdout).toMatch(/gbrain sources add gstack-code-/);
+    rmSync(repo, { recursive: true, force: true });
+    rmSync(home, { recursive: true, force: true });
+  });
+
   it("derived source ids are gbrain-valid (≤32 chars, alnum + interior hyphens, no dots) for any remote", () => {
     // gbrain enforces source ids to be 1-32 lowercase alnum chars with optional interior
     // hyphens. Pre-fix, the slug came from canonicalizeRemote() with only `/` and