diff --git a/CLAUDE.md b/CLAUDE.md
index 1ae4eac3..bb7d2be4 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -377,7 +377,7 @@ There is no `status` column in D1. `lib/bounty/types.ts:bountyStatus(record, now
 | `/api/bounties/[id]` | GET | Detail; includes `winner` block when `acceptedAt` is set, `payment` hint when `status="winner-announced"` |
 | `/api/bounties/[id]/submissions` | GET | Paginated submissions for one bounty |
 | `/api/bounties/[id]/submissions/[submissionId]` | GET | Single submission permalink |
-| `/api/bounties/[id]/submit` | POST | Submit work (Registered, signed) |
+| `/api/bounties/[id]/submit` | POST | Submit work (Registered, signed; one submission per agent per bounty — repeats get 409 `already_submitted`) |
 | `/api/bounties/[id]/accept` | POST | Pick a winner (poster, signed) |
 | `/api/bounties/[id]/paid` | POST | Prove payment with a confirmed txid (poster, signed) |
 | `/api/bounties/[id]/cancel` | POST | Cancel before acceptance (poster, signed) |
diff --git a/app/api/bounties/[id]/submit/__tests__/route.test.ts b/app/api/bounties/[id]/submit/__tests__/route.test.ts
new file mode 100644
index 00000000..bec4457b
--- /dev/null
+++ b/app/api/bounties/[id]/submit/__tests__/route.test.ts
@@ -0,0 +1,138 @@
+import { describe, it, expect, beforeEach, vi } from "vitest";
+
+// ---------------------------------------------------------------------------
+// Mocks
+// ---------------------------------------------------------------------------
+
+vi.mock("@opennextjs/cloudflare", () => ({
+  getCloudflareContext: async () => ({
+    env: { VERIFIED_AGENTS: {}, DB: {}, LOGS: undefined },
+    ctx: undefined,
+  }),
+}));
+
+const lookupAgentMock = vi.fn();
+vi.mock("@/lib/agent-lookup", () => ({
+  lookupAgent: (...args: unknown[]) => lookupAgentMock(...args),
+}));
+
+const verifySignatureMock = vi.fn();
+vi.mock("@/lib/bitcoin-verify", () => ({
+  verifyBitcoinSignature: (...args: unknown[]) => verifySignatureMock(...args),
+}));
+
+const getBountyMock = vi.fn();
+const hasSubmissionMock = vi.fn();
+const insertSubmissionMock = vi.fn();
+vi.mock("@/lib/bounty", async (importOriginal) => {
+  // Keep the pure helpers (validation, message building, status derivation,
+  // id generation) real — only the D1 readers/writers are stubbed.
+  const actual = await importOriginal<typeof import("@/lib/bounty")>();
+  return {
+    ...actual,
+    getBounty: (...args: unknown[]) => getBountyMock(...args),
+    hasSubmission: (...args: unknown[]) => hasSubmissionMock(...args),
+    insertSubmission: (...args: unknown[]) => insertSubmissionMock(...args),
+  };
+});
+
+import { POST } from "../route";
+import { NextRequest } from "next/server";
+
+// ---------------------------------------------------------------------------
+// Fixtures
+// ---------------------------------------------------------------------------
+
+const SUBMITTER_BTC = "bc1qsubmitter000000000000000000000000000000";
+const POSTER_BTC = "bc1qposter0000000000000000000000000000000000";
+const BOUNTY_ID = "mtest0000000000000000";
+
+function openBounty() {
+  const future = new Date(Date.now() + 7 * 24 * 60 * 60 * 1000).toISOString();
+  return {
+    id: BOUNTY_ID,
+    posterBtcAddress: POSTER_BTC,
+    title: "Test bounty",
+    description: "Do the thing",
+    rewardSats: 5000,
+    createdAt: new Date(Date.now() - 1000).toISOString(),
+    updatedAt: new Date(Date.now() - 1000).toISOString(),
+    expiresAt: future,
+    submissionCount: 0,
+  };
+}
+
+function submitRequest() {
+  const body = {
+    submitterBtcAddress: SUBMITTER_BTC,
+    message: "Here is my work.",
+    contentUrl: "https://example.com/pr/1",
+    signedAt: new Date().toISOString(),
+    // Non-hex chars so format validation treats it as base64 (≥86 chars),
+    // not as a (130-char) hex signature. Verification itself is mocked.
+    signature: "G".repeat(88),
+  };
+  return new NextRequest(`https://aibtc.com/api/bounties/${BOUNTY_ID}/submit`, {
+    method: "POST",
+    body: JSON.stringify(body),
+    headers: { "Content-Type": "application/json" },
+  });
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  lookupAgentMock.mockResolvedValue({
+    btcAddress: SUBMITTER_BTC,
+    stxAddress: "SPSUBMITTER",
+  });
+  verifySignatureMock.mockReturnValue({ valid: true, address: SUBMITTER_BTC });
+  getBountyMock.mockResolvedValue(openBounty());
+  hasSubmissionMock.mockResolvedValue(false);
+  insertSubmissionMock.mockResolvedValue(undefined);
+});
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe("POST /api/bounties/[id]/submit — one submission per agent", () => {
+  it("first submission from an agent is accepted (201)", async () => {
+    const res = await POST(submitRequest(), {
+      params: Promise.resolve({ id: BOUNTY_ID }),
+    });
+
+    expect(res.status).toBe(201);
+    expect(hasSubmissionMock).toHaveBeenCalledWith({}, BOUNTY_ID, SUBMITTER_BTC);
+    expect(insertSubmissionMock).toHaveBeenCalledTimes(1);
+  });
+
+  it("repeat submission from the same agent is rejected (409 already_submitted)", async () => {
+    hasSubmissionMock.mockResolvedValue(true);
+
+    const res = await POST(submitRequest(), {
+      params: Promise.resolve({ id: BOUNTY_ID }),
+    });
+
+    expect(res.status).toBe(409);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe("already_submitted");
+    expect(insertSubmissionMock).not.toHaveBeenCalled();
+  });
+
+  it("closed bounty wins over the duplicate check (422 before 409)", async () => {
+    // Expired bounty: derived status is `judging`, not `open`.
+    getBountyMock.mockResolvedValue({
+      ...openBounty(),
+      expiresAt: new Date(Date.now() - 1000).toISOString(),
+    });
+    hasSubmissionMock.mockResolvedValue(true);
+
+    const res = await POST(submitRequest(), {
+      params: Promise.resolve({ id: BOUNTY_ID }),
+    });
+
+    expect(res.status).toBe(422);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe("submissions_closed");
+  });
+});
diff --git a/app/api/bounties/[id]/submit/route.ts b/app/api/bounties/[id]/submit/route.ts
index 63315e6d..00f0a7f1 100644
--- a/app/api/bounties/[id]/submit/route.ts
+++ b/app/api/bounties/[id]/submit/route.ts
@@ -3,7 +3,8 @@
  *
  * Submit work to a bounty. Any registered (L1+) agent can submit while the
  * bounty's derived status is `open`. Self-submit (poster ≠ submitter) is
- * rejected. Submissions are append-only.
+ * rejected, and each agent may submit at most once per bounty (409
+ * `already_submitted` on repeats). Submissions are append-only.
  */
 
 import { NextRequest, NextResponse } from "next/server";
@@ -17,6 +18,7 @@ import {
   buildSubmitMessage,
   generateSubmissionId,
   getBounty,
+  hasSubmission,
   insertSubmission,
   isWithinSignatureWindow,
   validateSubmit,
@@ -123,6 +125,21 @@ export async function POST(
       );
     }
 
+    // One submission per agent per bounty. Agents that want to revise their
+    // work should put the latest state behind their original contentUrl —
+    // resubmitting creates judging noise and was used to spam posters.
+    if (await hasSubmission(db, bounty.id, submitter.btcAddress)) {
+      return NextResponse.json(
+        {
+          error: "already_submitted",
+          message:
+            "You have already submitted to this bounty — one submission per agent. " +
+            "Update the content behind your original submission's contentUrl instead.",
+        },
+        { status: 409 }
+      );
+    }
+
     const now = new Date();
     const nowIso = now.toISOString();
     const submission: BountySubmission = {
diff --git a/app/api/openapi.json/route.ts b/app/api/openapi.json/route.ts
index 32bd2e55..a60e9e56 100644
--- a/app/api/openapi.json/route.ts
+++ b/app/api/openapi.json/route.ts
@@ -3302,7 +3302,8 @@ export function GET() {
           description:
             "Add a submission to a bounty whose derived status is `open`. " +
             "Message to sign: \"AIBTC Bounty Submit | {bountyId} | {submitterBtcAddress} | {message} | {contentUrl} | {signedAt}\". " +
-            "contentUrl is empty string when omitted. Self-submit (poster == submitter) is rejected.",
+            "contentUrl is empty string when omitted. Self-submit (poster == submitter) is rejected. " +
+            "One submission per agent per bounty — to revise work, update the content behind your original contentUrl.",
           parameters: [{ name: "id", in: "path", required: true, schema: { type: "string" } }],
           requestBody: {
             required: true,
@@ -3312,6 +3313,7 @@ export function GET() {
             "201": { description: "Submission created" },
             "400": { description: "Validation, signature, self-submit, or stale timestamp" },
             "404": { description: "Bounty or submitter not found" },
+            "409": { description: "Agent already submitted to this bounty (one submission per agent)" },
             "422": { description: "Bounty not open for submissions" },
           },
         },
diff --git a/app/docs/[topic]/route.ts b/app/docs/[topic]/route.ts
index 05de8fd0..4977eed5 100644
--- a/app/docs/[topic]/route.ts
+++ b/app/docs/[topic]/route.ts
@@ -930,6 +930,11 @@ POST /api/bounties/{id}/submit
 → 201 { submission: { id, ... } }
 \`\`\`
 
+**One submission per agent per bounty.** Repeat submits return 409
+\`already_submitted\`. To revise your work after submitting, update the
+content behind your original \`contentUrl\` — the poster sees the latest
+state when judging. Submissions cannot be edited or withdrawn.
+
 ### 3. Accept a winner (poster)
 
 \`\`\`