diff --git a/src/config/config.session-write-lock-timeout.test.ts b/src/config/config.session-write-lock-timeout.test.ts
new file mode 100644
index 000000000..eecd1c115
--- /dev/null
+++ b/src/config/config.session-write-lock-timeout.test.ts
@@ -0,0 +1,87 @@
+import { describe, expect, it } from "vitest";
+import type { AgentDefaultsConfig } from "./types.agent-defaults.js";
+import { AgentDefaultsSchema } from "./zod-schema.agent-defaults.js";
+
+/**
+ * Helper to resolve session write lock timeout from config.
+ * Matches the pattern in session-write-lock.ts where default is 60_000ms.
+ */
+function resolveSessionWriteLockTimeout(config: {
+  agents?: { defaults?: { sessionWriteLockTimeoutMs?: number } };
+}): number {
+  return config.agents?.defaults?.sessionWriteLockTimeoutMs ?? 60_000;
+}
+
+describe("sessionWriteLockTimeoutMs config", () => {
+  it("uses default timeout when unset", () => {
+    const config = {};
+    expect(resolveSessionWriteLockTimeout(config)).toBe(60_000);
+  });
+
+  it("uses default timeout when agents.defaults is empty", () => {
+    const config = { agents: { defaults: {} } };
+    expect(resolveSessionWriteLockTimeout(config)).toBe(60_000);
+  });
+
+  it("uses custom timeout from config", () => {
+    const config = {
+      agents: {
+        defaults: {
+          sessionWriteLockTimeoutMs: 30_000,
+        },
+      },
+    };
+    expect(resolveSessionWriteLockTimeout(config)).toBe(30_000);
+  });
+
+  it("accepts large timeout values", () => {
+    const config = {
+      agents: {
+        defaults: {
+          sessionWriteLockTimeoutMs: 120_000,
+        },
+      },
+    };
+    expect(resolveSessionWriteLockTimeout(config)).toBe(120_000);
+  });
+
+  it("config type includes sessionWriteLockTimeoutMs field", () => {
+    const config: AgentDefaultsConfig = {
+      sessionWriteLockTimeoutMs: 45_000,
+    };
+    expect(config.sessionWriteLockTimeoutMs).toBe(45_000);
+  });
+
+  it("validates positive integer timeout via zod schema", () => {
+    const validConfig = { sessionWriteLockTimeoutMs: 30_000 };
+    const result = AgentDefaultsSchema.safeParse(validConfig);
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.sessionWriteLockTimeoutMs).toBe(30_000);
+    }
+  });
+
+  it("rejects negative timeout values", () => {
+    const invalidConfig = { sessionWriteLockTimeoutMs: -1000 };
+    const result = AgentDefaultsSchema.safeParse(invalidConfig);
+    expect(result.success).toBe(false);
+  });
+
+  it("rejects zero timeout value", () => {
+    const invalidConfig = { sessionWriteLockTimeoutMs: 0 };
+    const result = AgentDefaultsSchema.safeParse(invalidConfig);
+    expect(result.success).toBe(false);
+  });
+
+  it("rejects non-integer timeout values", () => {
+    const invalidConfig = { sessionWriteLockTimeoutMs: 1000.5 };
+    const result = AgentDefaultsSchema.safeParse(invalidConfig);
+    expect(result.success).toBe(false);
+  });
+
+  it("allows undefined/optional timeout", () => {
+    const config = {};
+    const result = AgentDefaultsSchema.safeParse(config);
+    expect(result.success).toBe(true);
+  });
+});