diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 95b940c97..5992513ac 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -172,6 +172,13 @@ importers:
       zod:
         specifier: ^4.3.6
         version: 4.3.6
+    optionalDependencies:
+      '@napi-rs/canvas':
+        specifier: ^0.1.88
+        version: 0.1.88
+      node-llama-cpp:
+        specifier: 3.15.0
+        version: 3.15.0(typescript@5.9.3)
     devDependencies:
       '@grammyjs/types':
         specifier: ^3.23.0
@@ -254,13 +261,6 @@ importers:
       wireit:
         specifier: ^0.14.12
         version: 0.14.12
-    optionalDependencies:
-      '@napi-rs/canvas':
-        specifier: ^0.1.88
-        version: 0.1.88
-      node-llama-cpp:
-        specifier: 3.15.0
-        version: 3.15.0(typescript@5.9.3)
 
   extensions/bluebubbles: {}
 
@@ -1328,7 +1328,6 @@ packages:
   '@lancedb/lancedb@0.23.0':
     resolution: {integrity: sha512-aYrIoEG24AC+wILCL57Ius/Y4yU+xFHDPKLvmjzzN4byAjzeIGF0TC86S5RBt4Ji+dxS7yIWV5Q/gE5/fybIFQ==}
     engines: {node: '>= 18'}
-    cpu: [x64, arm64]
     os: [darwin, linux, win32]
     peerDependencies:
       apache-arrow: '>=15.0.0 <=18.1.0'
diff --git a/src/agents/pi-embedded-runner/compact.ts b/src/agents/pi-embedded-runner/compact.ts
index 2dc4c5325..cf1191d2c 100644
--- a/src/agents/pi-embedded-runner/compact.ts
+++ b/src/agents/pi-embedded-runner/compact.ts
@@ -351,6 +351,7 @@ export async function compactEmbeddedPiSessionDirect(
 
     const sessionLock = await acquireSessionWriteLock({
       sessionFile: params.sessionFile,
+      timeoutMs: params.config?.agents?.defaults?.sessionWriteLockTimeoutMs,
     });
     try {
       await prewarmSessionFile(params.sessionFile);
diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index e83c3ae4a..dae26bb36 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -386,6 +386,7 @@ export async function runEmbeddedAttempt(
 
     const sessionLock = await acquireSessionWriteLock({
       sessionFile: params.sessionFile,
+      timeoutMs: params.config?.agents?.defaults?.sessionWriteLockTimeoutMs,
     });
 
     let sessionManager: ReturnType<typeof guardSessionManager> | undefined;
diff --git a/src/agents/session-write-lock.ts b/src/agents/session-write-lock.ts
index 82a2428da..24c88f5da 100644
--- a/src/agents/session-write-lock.ts
+++ b/src/agents/session-write-lock.ts
@@ -107,7 +107,9 @@ export async function acquireSessionWriteLock(params: {
   release: () => Promise<void>;
 }> {
   registerCleanupHandlers();
-  const timeoutMs = params.timeoutMs ?? 10_000;
+  // Default timeout increased from 10s to 60s to prevent premature termination
+  // when multiple subagents compete for session file locks (see issue #4355)
+  const timeoutMs = params.timeoutMs ?? 60_000;
   const staleMs = params.staleMs ?? 30 * 60 * 1000;
   const sessionFile = path.resolve(params.sessionFile);
   const sessionDir = path.dirname(sessionFile);
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);
+  });
+});
diff --git a/src/config/types.agent-defaults.ts b/src/config/types.agent-defaults.ts
index 9c6ce0211..e5cdb18bf 100644
--- a/src/config/types.agent-defaults.ts
+++ b/src/config/types.agent-defaults.ts
@@ -196,6 +196,8 @@ export type AgentDefaultsConfig = {
   };
   /** Max concurrent agent runs across all conversations. Default: 1 (sequential). */
   maxConcurrent?: number;
+  /** Timeout (ms) for acquiring per-session write lock. Default: 10000. */
+  sessionWriteLockTimeoutMs?: number;
   /** Sub-agent defaults (spawned via sessions_spawn). */
   subagents?: {
     /** Max concurrent sub-agent runs (global lane: "subagent"). Default: 1. */
diff --git a/src/config/zod-schema.agent-defaults.ts b/src/config/zod-schema.agent-defaults.ts
index a849078ed..906719132 100644
--- a/src/config/zod-schema.agent-defaults.ts
+++ b/src/config/zod-schema.agent-defaults.ts
@@ -123,6 +123,7 @@ export const AgentDefaultsSchema = z
     blockStreamingCoalesce: BlockStreamingCoalesceSchema.optional(),
     humanDelay: HumanDelaySchema.optional(),
     timeoutSeconds: z.number().int().positive().optional(),
+    sessionWriteLockTimeoutMs: z.number().int().positive().optional(),
     mediaMaxMb: z.number().positive().optional(),
     typingIntervalSeconds: z.number().int().positive().optional(),
     typingMode: z