diff --git a/packages/session-ui/src/components/message-part-task.test.ts b/packages/session-ui/src/components/message-part-task.test.ts
new file mode 100644
index 000000000..91d0310ff
--- /dev/null
+++ b/packages/session-ui/src/components/message-part-task.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, test } from "bun:test"
+import { findTaskSession, isAmicodeToolCall } from "./message-part-task"
+
+// Minimal session stub — only the fields findTaskSession reads.
+function session(overrides: {
+  id: string
+  parentID?: string
+  title?: string
+  archived?: number
+  created?: number
+}) {
+  return {
+    id: overrides.id,
+    parentID: overrides.parentID,
+    title: overrides.title as string, // intentionally allow undefined to match runtime
+    time: {
+      created: overrides.created ?? Date.now(),
+      updated: Date.now(),
+      archived: overrides.archived,
+    },
+  } as any
+}
+
+describe("findTaskSession", () => {
+  test("returns matching child session id", () => {
+    const sessions = [
+      session({ id: "child-1", parentID: "p1", title: "Fix bug @general", created: 100 }),
+    ]
+    expect(findTaskSession(sessions, "p1", "Fix bug", "general")).toBe("child-1")
+  })
+
+  test("returns undefined when no sessions match parentID", () => {
+    const sessions = [
+      session({ id: "child-1", parentID: "other", title: "Fix bug @general" }),
+    ]
+    expect(findTaskSession(sessions, "p1", "Fix bug", "general")).toBeUndefined()
+  })
+
+  test("skips archived sessions", () => {
+    const sessions = [
+      session({ id: "child-1", parentID: "p1", title: "Fix bug @general", archived: 1 }),
+    ]
+    expect(findTaskSession(sessions, "p1", "Fix bug", "general")).toBeUndefined()
+  })
+
+  test("returns most recently created when multiple match", () => {
+    const sessions = [
+      session({ id: "old", parentID: "p1", title: "Fix bug @general", created: 100 }),
+      session({ id: "new", parentID: "p1", title: "Fix bug @general", created: 200 }),
+    ]
+    expect(findTaskSession(sessions, "p1", "Fix bug", "general")).toBe("new")
+  })
+
+  test("does not crash when a session has undefined title", () => {
+    const sessions = [
+      session({ id: "no-title", parentID: "p1", title: undefined }),
+      session({ id: "titled", parentID: "p1", title: "Fix bug @general", created: 200 }),
+    ]
+    // Should not throw — must gracefully skip the untitled session
+    expect(() => findTaskSession(sessions, "p1", "Fix bug", "general")).not.toThrow()
+    expect(findTaskSession(sessions, "p1", "Fix bug", "general")).toBe("titled")
+  })
+
+  test("does not crash when all sessions have undefined title", () => {
+    const sessions = [
+      session({ id: "a", parentID: "p1", title: undefined }),
+      session({ id: "b", parentID: "p1", title: undefined }),
+    ]
+    expect(() => findTaskSession(sessions, "p1", "task", "general")).not.toThrow()
+    expect(findTaskSession(sessions, "p1", "task", "general")).toBeUndefined()
+  })
+
+  test("skips title filter when description is empty", () => {
+    const sessions = [
+      session({ id: "child-1", parentID: "p1", title: undefined }),
+    ]
+    // With empty description the startsWith filter is bypassed, but
+    // the includes(@agent) filter still runs on undefined title
+    expect(() => findTaskSession(sessions, "p1", "", "")).not.toThrow()
+    expect(findTaskSession(sessions, "p1", "", "")).toBe("child-1")
+  })
+
+  test("handles empty session list", () => {
+    expect(findTaskSession([], "p1", "desc", "general")).toBeUndefined()
+  })
+})
+
+describe("isAmicodeToolCall", () => {
+  test("returns true for amicode_ prefixed tool part", () => {
+    expect(isAmicodeToolCall({ type: "tool", tool: "amicode_solve" })).toBe(true)
+  })
+
+  test("returns false for non-tool part", () => {
+    expect(isAmicodeToolCall({ type: "text", tool: "amicode_solve" })).toBe(false)
+  })
+
+  test("returns false when part is undefined", () => {
+    expect(isAmicodeToolCall(undefined)).toBe(false)
+  })
+
+  test("does not crash when tool property is undefined", () => {
+    // A tool-type part with missing tool name — should not throw
+    expect(() => isAmicodeToolCall({ type: "tool", tool: undefined })).not.toThrow()
+    expect(isAmicodeToolCall({ type: "tool", tool: undefined })).toBe(false)
+  })
+
+  test("does not crash when tool property is null", () => {
+    expect(() => isAmicodeToolCall({ type: "tool", tool: null })).not.toThrow()
+    expect(isAmicodeToolCall({ type: "tool", tool: null })).toBe(false)
+  })
+
+  test("returns false for non-amicode tool", () => {
+    expect(isAmicodeToolCall({ type: "tool", tool: "bash" })).toBe(false)
+  })
+})
diff --git a/packages/session-ui/src/components/message-part-task.ts b/packages/session-ui/src/components/message-part-task.ts
new file mode 100644
index 000000000..0674bce99
--- /dev/null
+++ b/packages/session-ui/src/components/message-part-task.ts
@@ -0,0 +1,23 @@
+// Pure task-session matching helpers, extracted from message-part.tsx for
+// testability (same pattern as message-part-text.ts / message-part-groups.ts).
+// JSX-free, no DOM imports.
+
+/** Find the best matching child session for a Task tool-call display. */
+export function findTaskSession(
+  sessions: readonly { id: string; parentID?: string; title: string; time: { created?: number; archived?: number } }[],
+  parentID: string,
+  description: string,
+  agentName: string,
+): string | undefined {
+  return sessions
+    .filter((session) => session.parentID === parentID && !session.time?.archived)
+    .filter((session) => (description ? session.title?.startsWith(description) : true))
+    .filter((session) => (agentName ? session.title?.includes(`@${agentName}`) : true))
+    .sort((a, b) => (b.time.created ?? 0) - (a.time.created ?? 0))[0]?.id
+}
+
+/** Check whether a part-like object is an amicode_* tool call. */
+export function isAmicodeToolCall(part: { type?: string; tool?: unknown } | undefined | null): boolean {
+  if (!part || part.type !== "tool") return false
+  return typeof part.tool === "string" && part.tool.startsWith("amicode_")
+}
diff --git a/packages/session-ui/src/components/message-part.tsx b/packages/session-ui/src/components/message-part.tsx
index 4d932c801..7aba7e336 100644
--- a/packages/session-ui/src/components/message-part.tsx
+++ b/packages/session-ui/src/components/message-part.tsx
@@ -655,8 +655,8 @@ function taskSession(
   const agent = taskAgent(input.subagent_type, agents).name
   return (sessions ?? [])
     .filter((session) => session.parentID === parentID && !session.time?.archived)
-    .filter((session) => (description ? session.title.startsWith(description) : true))
-    .filter((session) => (agent ? session.title.includes(`@${agent}`) : true))
+    .filter((session) => (description ? session.title?.startsWith(description) : true))
+    .filter((session) => (agent ? session.title?.includes(`@${agent}`) : true))
     .sort((a, b) => (b.time.created ?? 0) - (a.time.created ?? 0))[0]?.id
 }
 
@@ -713,7 +713,7 @@ function index<T extends { id: string }>(items: readonly T[]) {
 // candidates; everything else (still running, errored, not amicode_*, no/
 // unparseable sentinel) gets `key: undefined` and can never merge.
 function amicodeReceiptCandidateKey(part: PartType | undefined): { key?: ReceiptKey; seq?: number } {
-  if (!part || part.type !== "tool" || !part.tool.startsWith("amicode_")) return {}
+  if (!part || part.type !== "tool" || typeof part.tool !== "string" || !part.tool.startsWith("amicode_")) return {}
   if (part.state.status !== "completed") return {}
   const sentinel = parseDiffSentinel(part.state.output)
   return { key: receiptRunKey(sentinel), seq: sentinel?.seq }