diff --git a/packages/db/package.json b/packages/db/package.json
index 2ca5f0c..2a86eaf 100644
--- a/packages/db/package.json
+++ b/packages/db/package.json
@@ -12,6 +12,7 @@
     "test": "vitest run"
   },
   "dependencies": {
+    "@devflow/core": "workspace:*",
     "drizzle-orm": "0.45.2",
     "pg": "8.20.0"
   }
diff --git a/packages/db/src/index.ts b/packages/db/src/index.ts
index d5323ad..be2c8b3 100644
--- a/packages/db/src/index.ts
+++ b/packages/db/src/index.ts
@@ -1,2 +1,3 @@
 export { createDbClient, type DbClient } from "./client.js";
+export * from "./repositories/transcript.js";
 export * from "./schema/index.js";
diff --git a/packages/db/src/repositories/transcript.test.ts b/packages/db/src/repositories/transcript.test.ts
new file mode 100644
index 0000000..1d71348
--- /dev/null
+++ b/packages/db/src/repositories/transcript.test.ts
@@ -0,0 +1,166 @@
+import { randomUUID } from "node:crypto";
+import { and, eq, inArray } from "drizzle-orm";
+import { afterEach, describe, expect, it } from "vitest";
+
+import { type DbClient, createDbClient } from "../client.js";
+import { runs, tuiSessions, tuiTranscriptChunks, workflowTemplates } from "../schema/index.js";
+import { TuiTranscriptRepository } from "./transcript.js";
+
+const testDatabaseUrl =
+  process.env.DATABASE_URL ?? "postgres://devflow:devflow@127.0.0.1:55432/devflow";
+
+describe("TuiTranscriptRepository", () => {
+  let client: DbClient | undefined;
+  const runIds: string[] = [];
+  const templateIds: string[] = [];
+
+  afterEach(async () => {
+    if (client === undefined) {
+      return;
+    }
+    if (runIds.length > 0) {
+      await client.db.delete(runs).where(inArray(runs.id, [...runIds]));
+    }
+    if (templateIds.length > 0) {
+      await client.db
+        .delete(workflowTemplates)
+        .where(inArray(workflowTemplates.id, [...templateIds]));
+    }
+    runIds.length = 0;
+    templateIds.length = 0;
+    await client.close();
+    client = undefined;
+  });
+
+  async function createSession() {
+    client = createDbClient(testDatabaseUrl);
+    const templateId = randomUUID();
+    const runId = randomUUID();
+    const sessionId = randomUUID();
+    templateIds.push(templateId);
+    runIds.push(runId);
+
+    await client.db.insert(workflowTemplates).values({
+      id: templateId,
+      name: `template-${templateId}`,
+      version: 1,
+      hash: `hash-${templateId}`,
+      definition: {},
+    });
+    await client.db.insert(runs).values({
+      id: runId,
+      templateId,
+      templateHash: `hash-${templateId}`,
+      state: "executing",
+      repoPath: `/tmp/devflow-${runId}`,
+      baseBranch: "main",
+      worktreeRoot: `/tmp/devflow-${runId}/main`,
+    });
+    await client.db.insert(tuiSessions).values({
+      id: sessionId,
+      runId,
+      roleId: "implementer",
+      backend: "fake",
+      cwd: `/tmp/devflow-${runId}/main`,
+      state: "READY",
+    });
+
+    return { db: client.db, sessionId };
+  }
+
+  it("appends transcript chunks idempotently and advances last_capture_seq", async () => {
+    const { db, sessionId } = await createSession();
+    const repository = new TuiTranscriptRepository(db);
+    const firstAt = new Date("2026-05-09T00:00:00.000Z");
+    const secondAt = new Date("2026-05-09T00:00:01.000Z");
+
+    await repository.append(sessionId, [
+      { seq: 1n, content: "first", capturedAt: firstAt },
+      { seq: 2n, content: "second", capturedAt: secondAt },
+    ]);
+    await repository.append(sessionId, [
+      { seq: 2n, content: "second", capturedAt: secondAt },
+      { seq: 3n, content: "third", capturedAt: new Date("2026-05-09T00:00:02.000Z") },
+    ]);
+
+    const rows = await db
+      .select({
+        seq: tuiTranscriptChunks.seq,
+        content: tuiTranscriptChunks.content,
+      })
+      .from(tuiTranscriptChunks)
+      .where(eq(tuiTranscriptChunks.sessionId, sessionId))
+      .orderBy(tuiTranscriptChunks.seq);
+    expect(rows).toEqual([
+      { seq: 1n, content: "first" },
+      { seq: 2n, content: "second" },
+      { seq: 3n, content: "third" },
+    ]);
+
+    const [session] = await db
+      .select({ lastCaptureSeq: tuiSessions.lastCaptureSeq })
+      .from(tuiSessions)
+      .where(eq(tuiSessions.id, sessionId));
+    expect(session?.lastCaptureSeq).toBe(3n);
+  });
+
+  it("rejects conflicting content for an existing transcript sequence", async () => {
+    const { db, sessionId } = await createSession();
+    const repository = new TuiTranscriptRepository(db);
+    const capturedAt = new Date("2026-05-09T00:00:00.000Z");
+
+    await repository.append(sessionId, [{ seq: 1n, content: "first", capturedAt }]);
+
+    await expect(
+      repository.append(sessionId, [{ seq: 1n, content: "different", capturedAt }]),
+    ).rejects.toMatchObject({ code: "transcript_seq_conflict" });
+
+    const rows = await db
+      .select()
+      .from(tuiTranscriptChunks)
+      .where(and(eq(tuiTranscriptChunks.sessionId, sessionId), eq(tuiTranscriptChunks.seq, 1n)));
+    expect(rows).toHaveLength(1);
+    expect(rows?.[0]?.content).toBe("first");
+  });
+
+  it("rejects sequence gaps before advancing last_capture_seq", async () => {
+    const { db, sessionId } = await createSession();
+    const repository = new TuiTranscriptRepository(db);
+
+    await expect(
+      repository.append(sessionId, [
+        { seq: 2n, content: "second", capturedAt: new Date("2026-05-09T00:00:01.000Z") },
+      ]),
+    ).rejects.toMatchObject({ code: "transcript_sequence_gap" });
+
+    const [session] = await db
+      .select({ lastCaptureSeq: tuiSessions.lastCaptureSeq })
+      .from(tuiSessions)
+      .where(eq(tuiSessions.id, sessionId));
+    expect(session?.lastCaptureSeq).toBe(0n);
+  });
+
+  it("surfaces concurrent conflicting content for the same transcript sequence", async () => {
+    const { db, sessionId } = await createSession();
+    const firstRepository = new TuiTranscriptRepository(db);
+    const secondRepository = new TuiTranscriptRepository(db);
+    const capturedAt = new Date("2026-05-09T00:00:00.000Z");
+
+    const results = await Promise.allSettled([
+      firstRepository.append(sessionId, [{ seq: 1n, content: "first", capturedAt }]),
+      secondRepository.append(sessionId, [{ seq: 1n, content: "different", capturedAt }]),
+    ]);
+
+    expect(results.filter((result) => result.status === "fulfilled")).toHaveLength(1);
+    const rejected = results.find((result) => result.status === "rejected");
+    expect(rejected).toMatchObject({
+      reason: expect.objectContaining({ code: "transcript_seq_conflict" }),
+    });
+
+    const rows = await db
+      .select()
+      .from(tuiTranscriptChunks)
+      .where(eq(tuiTranscriptChunks.sessionId, sessionId));
+    expect(rows).toHaveLength(1);
+  });
+});
diff --git a/packages/db/src/repositories/transcript.ts b/packages/db/src/repositories/transcript.ts
new file mode 100644
index 0000000..06be016
--- /dev/null
+++ b/packages/db/src/repositories/transcript.ts
@@ -0,0 +1,139 @@
+import { DevflowError } from "@devflow/core";
+import { and, eq, inArray, sql } from "drizzle-orm";
+
+import type { DbClient } from "../client.js";
+import { tuiSessions, tuiTranscriptChunks } from "../schema/index.js";
+
+export interface TranscriptChunkInput {
+  seq: bigint;
+  content: string;
+  capturedAt: Date;
+}
+
+export interface AppendTranscriptResult {
+  received: number;
+  inserted: number;
+  lastSeq: bigint | undefined;
+}
+
+type Database = DbClient["db"];
+
+export class TuiTranscriptRepository {
+  constructor(private readonly db: Database) {}
+
+  async append(
+    sessionId: string,
+    chunks: readonly TranscriptChunkInput[],
+  ): Promise<AppendTranscriptResult> {
+    if (chunks.length === 0) {
+      return { received: 0, inserted: 0, lastSeq: undefined };
+    }
+
+    const normalized = normalizeChunks(chunks);
+
+    return this.db.transaction(async (tx) => {
+      await tx.execute(
+        sql`SELECT pg_advisory_xact_lock(hashtext(${`devflow:tui-transcript:${sessionId}`}))`,
+      );
+
+      const [session] = await tx
+        .select({ lastCaptureSeq: tuiSessions.lastCaptureSeq })
+        .from(tuiSessions)
+        .where(eq(tuiSessions.id, sessionId));
+      if (session === undefined) {
+        throw new DevflowError("TUI session does not exist", {
+          class: "fatal",
+          code: "session_not_found",
+        });
+      }
+
+      const insertedRows = await tx
+        .insert(tuiTranscriptChunks)
+        .values(
+          normalized.map((chunk) => ({
+            sessionId,
+            seq: chunk.seq,
+            content: chunk.content,
+            capturedAt: chunk.capturedAt,
+          })),
+        )
+        .onConflictDoNothing({
+          target: [tuiTranscriptChunks.sessionId, tuiTranscriptChunks.seq],
+        })
+        .returning({ seq: tuiTranscriptChunks.seq });
+
+      const seqs = normalized.map((chunk) => chunk.seq);
+      const persistedRows = await tx
+        .select({
+          seq: tuiTranscriptChunks.seq,
+          content: tuiTranscriptChunks.content,
+        })
+        .from(tuiTranscriptChunks)
+        .where(
+          and(eq(tuiTranscriptChunks.sessionId, sessionId), inArray(tuiTranscriptChunks.seq, seqs)),
+        );
+      const persistedBySeq = new Map(persistedRows.map((row) => [row.seq, row.content]));
+
+      for (const chunk of normalized) {
+        const persisted = persistedBySeq.get(chunk.seq);
+        if (persisted !== chunk.content) {
+          throw new DevflowError("Transcript sequence already exists with different content", {
+            class: "fatal",
+            code: "transcript_seq_conflict",
+          });
+        }
+      }
+
+      const lastSeq = normalized.at(-1)?.seq;
+      const nextCaptureSeq = advanceContiguousCursor(session.lastCaptureSeq, normalized);
+      await tx
+        .update(tuiSessions)
+        .set({
+          lastCaptureSeq: nextCaptureSeq,
+        })
+        .where(eq(tuiSessions.id, sessionId));
+
+      return { received: chunks.length, inserted: insertedRows.length, lastSeq };
+    });
+  }
+}
+
+function normalizeChunks(chunks: readonly TranscriptChunkInput[]): TranscriptChunkInput[] {
+  const bySeq = new Map<bigint, TranscriptChunkInput>();
+  for (const chunk of chunks) {
+    if (chunk.seq <= 0n) {
+      throw new DevflowError("Transcript sequence must be positive", {
+        class: "fatal",
+        code: "transcript_sequence_invalid",
+      });
+    }
+
+    const existing = bySeq.get(chunk.seq);
+    if (existing !== undefined && existing.content !== chunk.content) {
+      throw new DevflowError("Duplicate transcript sequence has conflicting content", {
+        class: "fatal",
+        code: "transcript_seq_conflict",
+      });
+    }
+    bySeq.set(chunk.seq, chunk);
+  }
+
+  return [...bySeq.values()].sort((left, right) => Number(left.seq - right.seq));
+}
+
+function advanceContiguousCursor(current: bigint, chunks: readonly TranscriptChunkInput[]): bigint {
+  let cursor = current;
+  for (const chunk of chunks) {
+    if (chunk.seq <= cursor) {
+      continue;
+    }
+    if (chunk.seq !== cursor + 1n) {
+      throw new DevflowError("Transcript sequence cannot skip the capture cursor", {
+        class: "fatal",
+        code: "transcript_sequence_gap",
+      });
+    }
+    cursor = chunk.seq;
+  }
+  return cursor;
+}
diff --git a/packages/session/package.json b/packages/session/package.json
index d4dbc16..f29fc96 100644
--- a/packages/session/package.json
+++ b/packages/session/package.json
@@ -13,5 +13,8 @@
   },
   "dependencies": {
     "@devflow/core": "workspace:*"
+  },
+  "devDependencies": {
+    "@devflow/db": "workspace:*"
   }
 }
diff --git a/packages/session/src/index.ts b/packages/session/src/index.ts
index 644f756..d8479ea 100644
--- a/packages/session/src/index.ts
+++ b/packages/session/src/index.ts
@@ -1,2 +1,3 @@
 export * from "./adapter.js";
 export * from "./fake.js";
+export * from "./transcript.js";
diff --git a/packages/session/src/transcript.test.ts b/packages/session/src/transcript.test.ts
new file mode 100644
index 0000000..32e2bf6
--- /dev/null
+++ b/packages/session/src/transcript.test.ts
@@ -0,0 +1,154 @@
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+
+import type { PromptEnvelope } from "@devflow/core";
+import type { TuiTranscriptRepository } from "@devflow/db";
+
+import type { TranscriptChunk } from "./adapter.js";
+import { FakeSessionAdapter } from "./fake.js";
+import { captureAndPersistTranscript } from "./transcript.js";
+
+const runId = "00000000-0000-4000-8000-000000000001";
+
+function envelope(overrides: Partial<PromptEnvelope> = {}): PromptEnvelope {
+  return {
+    uuid: "00000000-0000-4000-8000-000000000010",
+    runId,
+    roleId: "implementer",
+    phaseKey: "implement",
+    attempt: 0,
+    expectedArtifact: join(mkdtempSync(join(tmpdir(), "devflow-transcript-artifact-")), "out.json"),
+    expectedSchema: "dev/spec@1",
+    dedupKey: "a".repeat(64),
+    instructions: "Scenario: timeout\nNo artifact needed",
+    ...overrides,
+  };
+}
+
+async function collectSink() {
+  const calls: Array<{ sessionId: string; chunks: TranscriptChunk[] }> = [];
+  return {
+    calls,
+    sink: {
+      async append(sessionId: string, chunks: readonly TranscriptChunk[]) {
+        calls.push({ sessionId, chunks: [...chunks] });
+      },
+    },
+  };
+}
+
+describe("captureAndPersistTranscript", () => {
+  const tempRoots: string[] = [];
+
+  afterEach(() => {
+    for (const root of tempRoots.splice(0)) {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it("captures adapter chunks after fromSeq and persists them to the sink", async () => {
+    const cwd = mkdtempSync(join(tmpdir(), "devflow-transcript-cwd-"));
+    tempRoots.push(cwd);
+    const adapter = new FakeSessionAdapter({ fixtureRoot: cwd, writeDelayMs: 0 });
+    const handle = await adapter.start({
+      runId,
+      roleId: "implementer",
+      backend: "fake",
+      cwd,
+    });
+    await adapter.sendPrompt(handle, envelope());
+    const { calls, sink } = await collectSink();
+
+    const result = await captureAndPersistTranscript({
+      adapter,
+      handle,
+      fromSeq: 1n,
+      sink,
+    });
+
+    expect(result).toEqual({ captured: 1, lastSeq: 2n });
+    expect(calls).toHaveLength(1);
+    expect(calls[0]).toMatchObject({
+      sessionId: handle.sessionId,
+      chunks: [
+        {
+          seq: 2n,
+          content: expect.stringContaining("timeout"),
+        },
+      ],
+    });
+  });
+
+  it("accepts the DB transcript repository as a sink contract", () => {
+    const repository = null as unknown as TuiTranscriptRepository;
+    const sink: Parameters<typeof captureAndPersistTranscript>[0]["sink"] = repository;
+
+    expect(sink).toBe(repository);
+  });
+
+  it("does not call the sink when there are no new chunks", async () => {
+    const cwd = mkdtempSync(join(tmpdir(), "devflow-transcript-cwd-"));
+    tempRoots.push(cwd);
+    const adapter = new FakeSessionAdapter({ fixtureRoot: cwd, writeDelayMs: 0 });
+    const handle = await adapter.start({
+      runId,
+      roleId: "implementer",
+      backend: "fake",
+      cwd,
+    });
+    const { calls, sink } = await collectSink();
+
+    const result = await captureAndPersistTranscript({
+      adapter,
+      handle,
+      fromSeq: 1n,
+      sink,
+    });
+
+    expect(result).toEqual({ captured: 0, lastSeq: 1n });
+    expect(calls).toEqual([]);
+  });
+
+  it("rejects non-monotonic adapter chunks before persistence", async () => {
+    const handle = { sessionId: "00000000-0000-4000-8000-000000000020" };
+    const now = new Date("2026-05-09T00:00:00.000Z");
+    const adapter = {
+      async *capture() {
+        yield { seq: 2n, content: "second", capturedAt: now };
+        yield { seq: 2n, content: "duplicate", capturedAt: now };
+      },
+    };
+    const { sink } = await collectSink();
+
+    await expect(
+      captureAndPersistTranscript({
+        adapter,
+        handle,
+        fromSeq: 1n,
+        sink,
+      }),
+    ).rejects.toMatchObject({ code: "transcript_sequence_invalid" });
+  });
+
+  it("rejects sequence gaps before advancing the capture watermark", async () => {
+    const handle = { sessionId: "00000000-0000-4000-8000-000000000021" };
+    const now = new Date("2026-05-09T00:00:00.000Z");
+    const adapter = {
+      async *capture() {
+        yield { seq: 3n, content: "third", capturedAt: now };
+      },
+    };
+    const { sink } = await collectSink();
+
+    await expect(
+      captureAndPersistTranscript({
+        adapter,
+        handle,
+        fromSeq: 1n,
+        sink,
+      }),
+    ).rejects.toMatchObject({ code: "transcript_sequence_gap" });
+  });
+});
diff --git a/packages/session/src/transcript.ts b/packages/session/src/transcript.ts
new file mode 100644
index 0000000..79a937a
--- /dev/null
+++ b/packages/session/src/transcript.ts
@@ -0,0 +1,51 @@
+import { DevflowError } from "@devflow/core";
+
+import type { SessionAdapter, SessionHandle, TranscriptChunk } from "./adapter.js";
+
+export interface TranscriptChunkSink {
+  append(sessionId: string, chunks: readonly TranscriptChunk[]): Promise<unknown>;
+}
+
+export interface CaptureAndPersistTranscriptInput {
+  adapter: Pick<SessionAdapter, "capture">;
+  handle: SessionHandle;
+  fromSeq: bigint;
+  sink: TranscriptChunkSink;
+}
+
+export interface CaptureAndPersistTranscriptResult {
+  captured: number;
+  lastSeq: bigint;
+}
+
+export async function captureAndPersistTranscript(
+  input: CaptureAndPersistTranscriptInput,
+): Promise<CaptureAndPersistTranscriptResult> {
+  const chunks: TranscriptChunk[] = [];
+  let lastSeq = input.fromSeq;
+
+  for await (const chunk of input.adapter.capture(input.handle, input.fromSeq)) {
+    if (chunk.seq <= lastSeq) {
+      throw new DevflowError("Transcript chunks must be strictly increasing", {
+        class: "fatal",
+        code: "transcript_sequence_invalid",
+      });
+    }
+    if (chunk.seq !== lastSeq + 1n) {
+      throw new DevflowError("Transcript chunks must be contiguous", {
+        class: "fatal",
+        code: "transcript_sequence_gap",
+      });
+    }
+    chunks.push(chunk);
+    lastSeq = chunk.seq;
+  }
+
+  if (chunks.length === 0) {
+    return { captured: 0, lastSeq: input.fromSeq };
+  }
+
+  await input.sink.append(input.handle.sessionId, chunks);
+
+  return { captured: chunks.length, lastSeq };
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 06ed763..e89ef8e 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -90,6 +90,9 @@ importers:
 
   packages/db:
     dependencies:
+      '@devflow/core':
+        specifier: workspace:*
+        version: link:../core
       drizzle-orm:
         specifier: 0.45.2
         version: 0.45.2(@types/pg@8.20.0)(pg@8.20.0)
@@ -102,6 +105,10 @@ importers:
       '@devflow/core':
         specifier: workspace:*
         version: link:../core
+    devDependencies:
+      '@devflow/db':
+        specifier: workspace:*
+        version: link:../db
 
 packages: