diff --git a/src/resources/extensions/gsd/auto/phases.ts b/src/resources/extensions/gsd/auto/phases.ts
index bda534c0b..b82f7e560 100644
--- a/src/resources/extensions/gsd/auto/phases.ts
+++ b/src/resources/extensions/gsd/auto/phases.ts
@@ -991,7 +991,12 @@ export async function runUnitPhase(
     unitId,
     prefs,
     buildSnapshotOpts: () => deps.buildSnapshotOpts(unitType, unitId),
-    buildRecoveryContext: () => ({}),
+    buildRecoveryContext: () => ({
+      basePath: s.basePath,
+      verbose: s.verbose,
+      currentUnitStartedAt: s.currentUnit?.startedAt ?? Date.now(),
+      unitRecoveryCount: s.unitRecoveryCount,
+    }),
     pauseAuto: deps.pauseAuto,
   });
 
diff --git a/src/resources/extensions/gsd/tests/stalled-tool-recovery.test.ts b/src/resources/extensions/gsd/tests/stalled-tool-recovery.test.ts
new file mode 100644
index 000000000..7d46c1128
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/stalled-tool-recovery.test.ts
@@ -0,0 +1,102 @@
+/**
+ * Regression test for #1855: Stalled tool detection crashes with
+ * "The path argument must be of type string. Received undefined"
+ *
+ * When a tool stalls in-flight for 10+ minutes, the idle watchdog fires
+ * recoverTimedOutUnit(). In auto/phases.ts, buildRecoveryContext was
+ * returning an empty object `{}`, so basePath was undefined. The recovery
+ * code passed undefined to readUnitRuntimeRecord → runtimePath → join(),
+ * which throws a TypeError. The session is permanently frozen because the
+ * error propagates into the idle watchdog catch handler but the unit
+ * promise is never resolved.
+ *
+ * This test calls recoverTimedOutUnit with an empty RecoveryContext (the
+ * bug) and verifies it crashes, then calls it with a valid RecoveryContext
+ * (the fix) and verifies it does not crash.
+ */
+
+import { mkdtempSync, mkdirSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { recoverTimedOutUnit, type RecoveryContext } from "../auto-timeout-recovery.ts";
+import { createTestContext } from './test-helpers.ts';
+
+const { assertTrue, report } = createTestContext();
+
+// Minimal mock for ExtensionContext — only the fields recoverTimedOutUnit touches.
+function makeMockCtx() {
+  return {
+    ui: {
+      notify: () => {},
+    },
+  } as any;
+}
+
+// Minimal mock for ExtensionAPI — only sendMessage is called during recovery.
+function makeMockPi() {
+  return {
+    sendMessage: () => {},
+  } as any;
+}
+
+// ═══ #1855: empty RecoveryContext (basePath undefined) crashes ════════════════
+
+{
+  console.log("\n=== #1855: recoverTimedOutUnit crashes when basePath is undefined ===");
+  const ctx = makeMockCtx();
+  const pi = makeMockPi();
+
+  // Simulate the bug: buildRecoveryContext returns {} (empty object).
+  // basePath is undefined, which causes join(undefined, ".gsd") to throw.
+  const emptyRctx = {} as RecoveryContext;
+
+  let crashed = false;
+  try {
+    await recoverTimedOutUnit(ctx, pi, "execute-task", "M001/S01/T01", "idle", emptyRctx);
+  } catch (err: any) {
+    crashed = true;
+    assertTrue(
+      err.message.includes("path") || err.message.includes("string") || err.code === "ERR_INVALID_ARG_TYPE",
+      `should crash with path/type error, got: ${err.message}`,
+    );
+  }
+  assertTrue(crashed, "should crash when basePath is undefined (reproduces #1855)");
+}
+
+// ═══ #1855: valid RecoveryContext does not crash ═════════════════════════════
+
+{
+  console.log("\n=== #1855: recoverTimedOutUnit succeeds with valid RecoveryContext ===");
+  const base = mkdtempSync(join(tmpdir(), "gsd-stalled-tool-test-"));
+  mkdirSync(join(base, ".gsd", "milestones", "M001", "slices", "S01", "tasks"), { recursive: true });
+  mkdirSync(join(base, ".gsd", "runtime", "units"), { recursive: true });
+
+  try {
+    const ctx = makeMockCtx();
+    const pi = makeMockPi();
+
+    const validRctx: RecoveryContext = {
+      basePath: base,
+      verbose: false,
+      currentUnitStartedAt: Date.now(),
+      unitRecoveryCount: new Map(),
+    };
+
+    let crashed = false;
+    let result: string | undefined;
+    try {
+      result = await recoverTimedOutUnit(ctx, pi, "execute-task", "M001/S01/T01", "idle", validRctx);
+    } catch (err: any) {
+      crashed = true;
+      console.error(`  Unexpected crash: ${err.message}`);
+    }
+    assertTrue(!crashed, "should not crash with valid basePath");
+    // With no runtime record on disk and recoveryAttempts=0, the function
+    // should attempt steering recovery (sendMessage) and return "recovered".
+    assertTrue(result === "recovered", `should return 'recovered', got '${result}'`);
+  } finally {
+    rmSync(base, { recursive: true, force: true });
+  }
+}
+
+report();