diff --git a/packages/agent/src/harness/session/jsonl-repo.ts b/packages/agent/src/harness/session/jsonl-repo.ts
index 1a08d4d3..f1f16522 100644
--- a/packages/agent/src/harness/session/jsonl-repo.ts
+++ b/packages/agent/src/harness/session/jsonl-repo.ts
@@ -85,6 +85,7 @@ export class JsonlSessionRepo implements JsonlSessionRepoApi {
 			cwd: options.cwd,
 			sessionId: id,
 			parentSessionPath: options.parentSessionPath,
+			metadata: options.metadata,
 		});
 		return toSession(storage);
 	}
@@ -150,6 +151,7 @@ export class JsonlSessionRepo implements JsonlSessionRepoApi {
 				cwd: options.cwd,
 				sessionId: id,
 				parentSessionPath: options.parentSessionPath ?? sourceMetadata.path,
+				metadata: options.metadata ?? sourceMetadata.metadata,
 			},
 		);
 		for (const entry of forkedEntries) {
diff --git a/packages/agent/src/harness/session/jsonl-storage.ts b/packages/agent/src/harness/session/jsonl-storage.ts
index 912673ff..b4111572 100644
--- a/packages/agent/src/harness/session/jsonl-storage.ts
+++ b/packages/agent/src/harness/session/jsonl-storage.ts
@@ -12,6 +12,7 @@ interface SessionHeader {
 	timestamp: string;
 	cwd: string;
 	parentSession?: string;
+	metadata?: Record<string, unknown>;
 }
 
 function updateLabelCache(labelsById: Map<string, string>, entry: SessionTreeEntry): void {
@@ -75,6 +76,12 @@ function parseHeaderLine(line: string, filePath: string): SessionHeader {
 	if (header.parentSession !== undefined && typeof header.parentSession !== "string") {
 		throw invalidSession(filePath, "session header parentSession must be a string");
 	}
+	if (
+		header.metadata !== undefined &&
+		(typeof header.metadata !== "object" || header.metadata === null || Array.isArray(header.metadata))
+	) {
+		throw invalidSession(filePath, "session header metadata must be an object");
+	}
 	return {
 		type: "session",
 		version: 3,
@@ -82,6 +89,7 @@ function parseHeaderLine(line: string, filePath: string): SessionHeader {
 		timestamp: header.timestamp,
 		cwd: header.cwd,
 		parentSession: header.parentSession,
+		metadata: header.metadata,
 	};
 }
 
@@ -127,6 +135,7 @@ function headerToSessionMetadata(header: SessionHeader, path: string): JsonlSess
 		cwd: header.cwd,
 		path,
 		parentSessionPath: header.parentSession,
+		metadata: header.metadata,
 	};
 }
 
@@ -205,6 +214,7 @@ export class JsonlSessionStorage implements SessionStorage<JsonlSessionMetadata>
 			cwd: string;
 			sessionId: string;
 			parentSessionPath?: string;
+			metadata?: Record<string, unknown>;
 		},
 	): Promise<JsonlSessionStorage> {
 		const header: SessionHeader = {
@@ -214,6 +224,7 @@ export class JsonlSessionStorage implements SessionStorage<JsonlSessionMetadata>
 			timestamp: new Date().toISOString(),
 			cwd: options.cwd,
 			parentSession: options.parentSessionPath,
+			metadata: options.metadata,
 		};
 		getFileSystemResultOrThrow(
 			await fs.writeFile(filePath, `${JSON.stringify(header)}\n`),
diff --git a/packages/agent/src/harness/types.ts b/packages/agent/src/harness/types.ts
index 29048e61..3f965550 100644
--- a/packages/agent/src/harness/types.ts
+++ b/packages/agent/src/harness/types.ts
@@ -435,6 +435,7 @@ export interface JsonlSessionMetadata extends SessionMetadata {
 	cwd: string;
 	path: string;
 	parentSessionPath?: string;
+	metadata?: Record<string, unknown>;
 }
 
 export interface SessionStorage<TMetadata extends SessionMetadata = SessionMetadata> {
@@ -480,6 +481,7 @@ export interface SessionRepo<
 export interface JsonlSessionCreateOptions extends SessionCreateOptions {
 	cwd: string;
 	parentSessionPath?: string;
+	metadata?: Record<string, unknown>;
 }
 
 export interface JsonlSessionListOptions {
diff --git a/packages/agent/test/harness/repo.test.ts b/packages/agent/test/harness/repo.test.ts
index b768fc1a..009fb0ce 100644
--- a/packages/agent/test/harness/repo.test.ts
+++ b/packages/agent/test/harness/repo.test.ts
@@ -65,4 +65,28 @@ describe("JsonlSessionRepo", () => {
 		expect(existsSync(sourceMetadata.path)).toBe(false);
 		await expect(repo.open(sourceMetadata)).rejects.toThrow("Session not found");
 	});
+
+	it("persists header metadata through create, list, and fork", async () => {
+		const root = createTempDir();
+		const env = new NodeExecutionEnv({ cwd: root });
+		const repo = new JsonlSessionRepo({ fs: env, sessionsRoot: root });
+		const source = await repo.create({
+			cwd: "/tmp/source",
+			id: "source-session",
+			metadata: { profile: "reviewer" },
+		});
+		const sourceMetadata = await source.getMetadata();
+		expect(sourceMetadata.metadata).toEqual({ profile: "reviewer" });
+		expect((await repo.list({ cwd: "/tmp/source" })).map((listed) => listed.metadata)).toEqual([
+			{ profile: "reviewer" },
+		]);
+		const fork = await repo.fork(sourceMetadata, { cwd: "/tmp/target", id: "fork-session" });
+		expect((await fork.getMetadata()).metadata).toEqual({ profile: "reviewer" });
+		const overridden = await repo.fork(sourceMetadata, {
+			cwd: "/tmp/target",
+			id: "overridden-session",
+			metadata: { profile: "writer" },
+		});
+		expect((await overridden.getMetadata()).metadata).toEqual({ profile: "writer" });
+	});
 });
diff --git a/packages/agent/test/harness/storage.test.ts b/packages/agent/test/harness/storage.test.ts
index e8383ce4..07f07df2 100644
--- a/packages/agent/test/harness/storage.test.ts
+++ b/packages/agent/test/harness/storage.test.ts
@@ -186,6 +186,48 @@ describe("JsonlSessionStorage", () => {
 		expect(await loadJsonlSessionMetadata(env, filePath)).toEqual(metadata);
 	});
 
+	it("round-trips custom header metadata", async () => {
+		const dir = createTempDir();
+		const env = new NodeExecutionEnv({ cwd: dir });
+		const filePath = join(dir, "session.jsonl");
+		const storage = await JsonlSessionStorage.create(env, filePath, {
+			cwd: dir,
+			sessionId: "session-1",
+			metadata: { profile: "reviewer" },
+		});
+		expect((await storage.getMetadata()).metadata).toEqual({ profile: "reviewer" });
+		const loaded = await JsonlSessionStorage.open(env, filePath);
+		expect((await loaded.getMetadata()).metadata).toEqual({ profile: "reviewer" });
+		expect((await loadJsonlSessionMetadata(env, filePath)).metadata).toEqual({ profile: "reviewer" });
+	});
+
+	it("omits header metadata when not provided", async () => {
+		const dir = createTempDir();
+		const env = new NodeExecutionEnv({ cwd: dir });
+		const filePath = join(dir, "session.jsonl");
+		await JsonlSessionStorage.create(env, filePath, { cwd: dir, sessionId: "session-1" });
+		expect(JSON.parse(readFileSync(filePath, "utf8").trim())).not.toHaveProperty("metadata");
+		expect((await loadJsonlSessionMetadata(env, filePath)).metadata).toBeUndefined();
+	});
+
+	it("throws for non-object header metadata", async () => {
+		const dir = createTempDir();
+		const env = new NodeExecutionEnv({ cwd: dir });
+		const filePath = join(dir, "session.jsonl");
+		const header = {
+			type: "session",
+			version: 3,
+			id: "session-1",
+			timestamp: "2026-01-01T00:00:00.000Z",
+			cwd: dir,
+			metadata: "profile",
+		};
+		writeFileSync(filePath, `${JSON.stringify(header)}\n`);
+		await expect(JsonlSessionStorage.open(env, filePath)).rejects.toThrow(
+			"session header metadata must be an object",
+		);
+	});
+
 	it("loads existing entries and reconstructs leaf", async () => {
 		const dir = createTempDir();
 		const env = new NodeExecutionEnv({ cwd: dir });