diff --git a/src/providers/constants.ts b/src/providers/constants.ts
index 0e58641..749f978 100644
--- a/src/providers/constants.ts
+++ b/src/providers/constants.ts
@@ -30,8 +30,8 @@ export const CLAUDE_REQUIRED_BETA_HEADERS = [
   CLAUDE_INTERLEAVED_THINKING_BETA_HEADER,
   CLAUDE_FINE_GRAINED_TOOL_STREAMING_BETA_HEADER,
 ] as const;
-// https://github.com/badlogic/pi-mono/blob/5c0ec26c28c918c5301f218e8c13fcc540d8e3a4/packages/ai/src/providers/anthropic.ts#L537
-export const CLAUDE_CLI_USER_AGENT = "claude-cli/2.1.2 (external, cli)";
+// Claude Fable 5.1 requires Claude Code 2.1.251 or newer.
+export const CLAUDE_CLI_USER_AGENT = "claude-cli/2.1.251 (external, cli)";
 // https://github.com/badlogic/pi-mono/blob/5c0ec26c28c918c5301f218e8c13fcc540d8e3a4/packages/ai/src/providers/anthropic.ts#L581-L586
 export const CLAUDE_SYSTEM_IDENTITY =
   "You are Claude Code, Anthropic's official CLI for Claude.";
diff --git a/src/providers/proxies/claude-proxy.ts b/src/providers/proxies/claude-proxy.ts
index 80152c9..d8302d1 100644
--- a/src/providers/proxies/claude-proxy.ts
+++ b/src/providers/proxies/claude-proxy.ts
@@ -655,10 +655,9 @@ export const prepareClaudeProxyRequest = (
   // https://github.com/badlogic/pi-mono/blob/5c0ec26c28c918c5301f218e8c13fcc540d8e3a4/packages/ai/src/providers/anthropic.ts#L525-L538
   input.headers.set("authorization", `Bearer ${input.accessToken}`);
   input.headers.set("anthropic-beta", mergedBetas);
-  input.headers.set(
-    "user-agent",
-    input.metadata?.userAgent ?? CLAUDE_CLI_USER_AGENT
-  );
+  // Keep existing accounts on the minimum supported Claude Code version even
+  // when their persisted metadata still contains an older user agent.
+  input.headers.set("user-agent", CLAUDE_CLI_USER_AGENT);
   input.headers.set("x-app", "cli");
 
   const transformedPayload = transformClaudeRequestPayload(
diff --git a/tests/providers/proxy-contract.test.ts b/tests/providers/proxy-contract.test.ts
index 5377140..5ee1d1f 100644
--- a/tests/providers/proxy-contract.test.ts
+++ b/tests/providers/proxy-contract.test.ts
@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, test } from "bun:test";
 
 import {
+  CLAUDE_CLI_USER_AGENT,
   CLAUDE_REQUIRED_BETA_HEADERS,
   CLAUDE_SYSTEM_IDENTITY,
   CODEX_ACCOUNT_ID_HEADER,
@@ -12,6 +13,7 @@ import {
   COPILOT_VISION_HEADER,
 } from "../../src/providers/constants";
 import type {
+  ClaudeAccountMetadata,
   CodexAccountMetadata,
   CopilotAccountMetadata,
 } from "../../src/providers/metadata";
@@ -2388,6 +2390,18 @@ describe("proxy contract: copilot", () => {
 });
 
 describe("proxy contract: claude", () => {
+  const legacyClaudeMetadata: ClaudeAccountMetadata = {
+    provider: "claude",
+    tokenType: null,
+    scope: null,
+    oauthMode: "max",
+    oauthHost: "claude.ai",
+    betaHeaders: [...CLAUDE_REQUIRED_BETA_HEADERS],
+    userAgent: "claude-cli/2.1.2 (external, cli)",
+    systemIdentity: CLAUDE_SYSTEM_IDENTITY,
+    toolPrefix: "mcp_",
+  };
+
   const prepareClaudeUsageRequest = (
     onTokenUsage?: ((usage: TokenUsage) => void) | null
   ) =>
@@ -2434,7 +2448,7 @@ describe("proxy contract: claude", () => {
       bodyText: JSON.stringify(requestBody),
       bodyJson: requestBody,
       accessToken: "claude-token",
-      metadata: null,
+      metadata: legacyClaudeMetadata,
     });
 
     const transformed = JSON.parse(result.bodyText) as {
@@ -2445,6 +2459,7 @@ describe("proxy contract: claude", () => {
     };
 
     expect(headers.get("authorization")).toBe("Bearer claude-token");
+    expect(headers.get("user-agent")).toBe(CLAUDE_CLI_USER_AGENT);
     expect(headers.get("anthropic-beta")).toBe(
       [...CLAUDE_REQUIRED_BETA_HEADERS, "custom-beta"].join(",")
     );