diff --git a/packages/coding-agent/src/core/session-manager.ts b/packages/coding-agent/src/core/session-manager.ts
index 57f9655ba..89431f2a3 100644
--- a/packages/coding-agent/src/core/session-manager.ts
+++ b/packages/coding-agent/src/core/session-manager.ts
@@ -1189,10 +1189,20 @@ export class SessionManager {
 	}
 
 	private _appendEntry(entry: SessionEntry): void {
+		const previousLeafId = this.leafId;
 		this.fileEntries.push(entry);
 		this.byId.set(entry.id, entry);
 		this.leafId = entry.id;
-		this._persist(entry);
+		try {
+			this._persist(entry);
+		} catch (error) {
+			// Keep memory in line with the file: an entry that was not written must not become the
+			// parent of later entries, or the file would hold entries pointing to a missing parent.
+			this.fileEntries.pop();
+			this.byId.delete(entry.id);
+			this.leafId = previousLeafId;
+			throw error;
+		}
 	}
 
 	/** Append a message as child of current leaf, then advance leaf. Returns entry id.
diff --git a/packages/coding-agent/test/session-manager/file-operations.test.ts b/packages/coding-agent/test/session-manager/file-operations.test.ts
index 8353ccdb7..ac6441490 100644
--- a/packages/coding-agent/test/session-manager/file-operations.test.ts
+++ b/packages/coding-agent/test/session-manager/file-operations.test.ts
@@ -1,6 +1,7 @@
 import { constants as bufferConstants } from "buffer";
 import {
 	appendFileSync,
+	chmodSync,
 	closeSync,
 	existsSync,
 	mkdirSync,
@@ -471,4 +472,42 @@ describe("SessionManager session file creation", () => {
 
 		expect(readSessionFileRoles(session.getSessionFile()!)).toEqual(["session", "user", "custom", "assistant"]);
 	});
+
+	// #10166, #10661, #11460: a failed write must not leave later entries pointing to a missing parent
+	it("does not keep an entry in memory when writing it fails", () => {
+		const session = SessionManager.create(tempDir, tempDir);
+		const firstId = session.appendMessage(userMsg("first question"));
+		const file = session.getSessionFile()!;
+
+		chmodSync(file, 0o444);
+		expect(() => session.appendMessage(assistantMsg("lost answer"))).toThrow();
+		expect(session.getLeafId()).toBe(firstId);
+		expect(session.getEntries().map((entry) => entry.id)).toEqual([firstId]);
+
+		chmodSync(file, 0o644);
+		const secondId = session.appendMessage(assistantMsg("second answer"));
+		expect(session.getEntry(secondId)?.parentId).toBe(firstId);
+
+		const reopened = SessionManager.open(file, tempDir);
+		expect(reopened.getBranch().map((entry) => entry.id)).toEqual([firstId, secondId]);
+	});
+
+	it("does not keep an entry in memory when creating the file fails", () => {
+		const sessionDir = join(tempDir, "sessions");
+		mkdirSync(sessionDir);
+		const session = SessionManager.create(tempDir, sessionDir);
+		const modelChangeId = session.appendModelChange("anthropic", "claude-sonnet-4-5");
+
+		chmodSync(sessionDir, 0o555);
+		try {
+			expect(() => session.appendMessage(userMsg("first question"))).toThrow();
+		} finally {
+			chmodSync(sessionDir, 0o755);
+		}
+		expect(session.getLeafId()).toBe(modelChangeId);
+		expect(session.getEntries().map((entry) => entry.id)).toEqual([modelChangeId]);
+
+		session.appendMessage(userMsg("second question"));
+		expect(readSessionFileRoles(session.getSessionFile()!)).toEqual(["session", "model_change", "user"]);
+	});
 });