diff --git a/src/localRecoveryCli.test.ts b/src/localRecoveryCli.test.ts
new file mode 100644
index 00000000..dade652a
--- /dev/null
+++ b/src/localRecoveryCli.test.ts
@@ -0,0 +1,146 @@
+import { describe, expect, spyOn, test } from 'bun:test'
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { Readable } from 'node:stream'
+import { createSandboxedTestEnvironment } from '../scripts/pr/test-environment.js'
+import { run } from './localRecoveryCli.js'
+
+describe('local recovery Anthropic base URL compatibility', () => {
+  for (const mode of ['print', 'interactive'] as const) {
+    for (const baseUrl of [undefined, 'https://provider.example/anthropic/v1/']) {
+      test(`${mode} entry point uses ${baseUrl ?? 'the default Anthropic endpoint'} with supplied input`, async () => {
+        const sandboxHome = await mkdtemp(join(tmpdir(), 'recovery-entry-point-'))
+        const previousEnv = { ...process.env }
+        const testEnv = createSandboxedTestEnvironment(sandboxHome, {
+          ANTHROPIC_API_KEY: 'sk-test-recovery',
+          ANTHROPIC_MODEL: 'claude-sonnet-4-6',
+          ...(baseUrl ? { ANTHROPIC_BASE_URL: baseUrl } : {}),
+        })
+        for (const key of Object.keys(process.env)) delete process.env[key]
+        Object.assign(process.env, testEnv)
+        let stdout = ''
+        let stderr = ''
+        const requests: Array<{ url: string; body: unknown }> = []
+        const stdoutSpy = spyOn(process.stdout, 'write').mockImplementation(chunk => {
+          stdout += String(chunk)
+          return true
+        })
+        const stderrSpy = spyOn(process.stderr, 'write').mockImplementation(chunk => {
+          stderr += String(chunk)
+          return true
+        })
+        const fetchSpy = spyOn(globalThis, 'fetch').mockImplementation(async (input, init) => {
+          requests.push({
+            url: input instanceof Request ? input.url : String(input),
+            body: JSON.parse(String(init?.body)),
+          })
+          return Response.json({
+            id: 'msg-recovery-entry-point',
+            type: 'message',
+            role: 'assistant',
+            model: 'claude-sonnet-4-6',
+            content: [{ type: 'text', text: 'entry point succeeded' }],
+            stop_reason: 'end_turn',
+            stop_sequence: null,
+            usage: { input_tokens: 1, output_tokens: 1 },
+          })
+        })
+
+        try {
+          await run(
+            mode === 'print' ? ['--print'] : [],
+            Readable.from([mode === 'print' ? 'hello\n' : 'hello\n/exit\n']),
+          )
+          expect(stderr).toBe('')
+          expect(stdout).toContain('entry point succeeded')
+          expect(requests).toEqual([{
+            url: baseUrl ? 'https://provider.example/anthropic/v1/messages' : 'https://api.anthropic.com/v1/messages',
+            body: expect.objectContaining({
+              model: 'claude-sonnet-4-6',
+              messages: [{ role: 'user', content: 'hello' }],
+            }),
+          }])
+        } finally {
+          fetchSpy.mockRestore()
+          stdoutSpy.mockRestore()
+          stderrSpy.mockRestore()
+          for (const key of Object.keys(process.env)) delete process.env[key]
+          Object.assign(process.env, previousEnv)
+          await rm(sandboxHome, { recursive: true, force: true })
+        }
+      })
+    }
+  }
+
+  for (const mode of ['print', 'interactive'] as const) {
+    for (const basePath of ['/anthropic', '/anthropic/v1/']) {
+      test(`${mode} sends messages through ${basePath} without duplicating the API version`, async () => {
+        const sandboxHome = await mkdtemp(join(tmpdir(), 'recovery-base-url-'))
+        const requests: Array<{ path: string; body: unknown }> = []
+        const upstream = Bun.serve({
+          hostname: '127.0.0.1',
+          port: 0,
+          async fetch(request) {
+            const pathname = new URL(request.url).pathname
+            requests.push({ path: pathname, body: await request.json() })
+            if (pathname !== '/anthropic/v1/messages') {
+              return Response.json({ error: { type: 'not_found_error', message: 'wrong endpoint' } }, { status: 404 })
+            }
+            return Response.json({
+              id: 'msg-recovery-test',
+              type: 'message',
+              role: 'assistant',
+              model: 'claude-sonnet-4-6',
+              content: [{ type: 'text', text: 'recovery request succeeded' }],
+              stop_reason: 'end_turn',
+              stop_sequence: null,
+              usage: { input_tokens: 1, output_tokens: 1 },
+            })
+          },
+        })
+        let child: ReturnType<typeof Bun.spawn> | undefined
+
+        try {
+          child = Bun.spawn([
+            process.execPath,
+            '--no-env-file',
+            join(import.meta.dir, 'localRecoveryCli.ts'),
+            ...(mode === 'print' ? ['--print', 'hello'] : []),
+          ], {
+            cwd: join(import.meta.dir, '..'),
+            env: createSandboxedTestEnvironment(sandboxHome, {
+              ANTHROPIC_API_KEY: 'sk-test-recovery',
+              ANTHROPIC_BASE_URL: `http://127.0.0.1:${upstream.port}${basePath}`,
+              ANTHROPIC_MODEL: 'claude-sonnet-4-6',
+              API_TIMEOUT_MS: '2000',
+            }),
+            stdin: mode === 'interactive' ? new Blob(['hello\n/exit\n']) : 'ignore',
+            stdout: 'pipe',
+            stderr: 'pipe',
+          })
+          const [stdout, stderr, exitCode] = await Promise.all([
+            new Response(child.stdout).text(),
+            new Response(child.stderr).text(),
+            child.exited,
+          ])
+
+          expect(stderr).toBe('')
+          expect(exitCode).toBe(0)
+          expect(stdout).toContain('recovery request succeeded')
+          expect(requests).toEqual([{
+            path: '/anthropic/v1/messages',
+            body: expect.objectContaining({
+              model: 'claude-sonnet-4-6',
+              messages: [{ role: 'user', content: 'hello' }],
+            }),
+          }])
+        } finally {
+          child?.kill()
+          upstream.stop(true)
+          await rm(sandboxHome, { recursive: true, force: true })
+        }
+      })
+    }
+  }
+})
diff --git a/src/localRecoveryCli.ts b/src/localRecoveryCli.ts
index 69b94baf..3cd347f9 100644
--- a/src/localRecoveryCli.ts
+++ b/src/localRecoveryCli.ts
@@ -1,8 +1,10 @@
 import Anthropic from '@anthropic-ai/sdk'
 import { readFileSync } from 'fs'
 import { createInterface } from 'readline'
+import { normalizeAnthropicBaseUrl } from './services/api/anthropicBaseUrl.js'
 
 type OutputFormat = 'text' | 'json'
+type RecoveryInput = NodeJS.ReadableStream & { isTTY?: boolean }
 
 function printHelp(): void {
   process.stdout.write(
@@ -105,10 +107,10 @@ function parseArgs(argv: string[]) {
   }
 }
 
-async function readPromptFromStdin(): Promise<string> {
-  if (process.stdin.isTTY) return ''
+async function readPromptFromStdin(input: RecoveryInput): Promise<string> {
+  if (input.isTTY) return ''
   const chunks: Buffer[] = []
-  for await (const chunk of process.stdin) {
+  for await (const chunk of input) {
     chunks.push(Buffer.isBuffer(chunk) ? chunk : Buffer.from(String(chunk)))
   }
   return Buffer.concat(chunks).toString('utf8').trim()
@@ -124,8 +126,11 @@ function getSystemPrompt(
   return systemPrompt ?? appendSystemPrompt
 }
 
-async function run(): Promise<void> {
-  const parsed = parseArgs(process.argv.slice(2))
+export async function run(
+  argv = process.argv.slice(2),
+  input: RecoveryInput = process.stdin,
+): Promise<void> {
+  const parsed = parseArgs(argv)
 
   if (parsed.command === 'help') {
     printHelp()
@@ -137,11 +142,11 @@ async function run(): Promise<void> {
   }
 
   if (!parsed.print) {
-    await runInteractive(parsed)
+    await runInteractive(parsed, input)
     return
   }
 
-  const prompt = parsed.prompt || (await readPromptFromStdin())
+  const prompt = parsed.prompt || (await readPromptFromStdin(input))
   if (!prompt) {
     process.stderr.write('Error: prompt is required\n')
     process.exitCode = 1
@@ -172,7 +177,9 @@ async function run(): Promise<void> {
   const client = new Anthropic({
     apiKey: apiKey ?? undefined,
     authToken: authToken ?? undefined,
-    baseURL: process.env.ANTHROPIC_BASE_URL || undefined,
+    baseURL: process.env.ANTHROPIC_BASE_URL
+      ? normalizeAnthropicBaseUrl(process.env.ANTHROPIC_BASE_URL)
+      : undefined,
     timeout: parseInt(process.env.API_TIMEOUT_MS || String(600_000), 10),
     maxRetries: 0,
   })
@@ -201,7 +208,7 @@ async function runInteractive(parsed: {
   model?: string
   systemPrompt?: string
   appendSystemPrompt?: string
-}): Promise<void> {
+}, input: RecoveryInput): Promise<void> {
   const apiKey = process.env.ANTHROPIC_API_KEY
   const authToken = process.env.ANTHROPIC_AUTH_TOKEN
   if (!apiKey && !authToken) {
@@ -226,7 +233,9 @@ async function runInteractive(parsed: {
   const client = new Anthropic({
     apiKey: apiKey ?? undefined,
     authToken: authToken ?? undefined,
-    baseURL: process.env.ANTHROPIC_BASE_URL || undefined,
+    baseURL: process.env.ANTHROPIC_BASE_URL
+      ? normalizeAnthropicBaseUrl(process.env.ANTHROPIC_BASE_URL)
+      : undefined,
     timeout: parseInt(process.env.API_TIMEOUT_MS || String(600_000), 10),
     maxRetries: 0,
   })
@@ -234,7 +243,7 @@ async function runInteractive(parsed: {
   const system = getSystemPrompt(parsed.systemPrompt, parsed.appendSystemPrompt)
   const messages: Array<{ role: 'user' | 'assistant'; content: string }> = []
   const rl = createInterface({
-    input: process.stdin,
+    input,
     output: process.stdout,
     prompt: 'you> ',
   })
@@ -284,8 +293,10 @@ async function runInteractive(parsed: {
   }
 }
 
-void run().catch(error => {
-  const message = error instanceof Error ? error.stack || error.message : String(error)
-  process.stderr.write(`${message}\n`)
-  process.exitCode = 1
-})
+if (import.meta.main) {
+  void run().catch(error => {
+    const message = error instanceof Error ? error.stack || error.message : String(error)
+    process.stderr.write(`${message}\n`)
+    process.exitCode = 1
+  })
+}
diff --git a/src/server/__tests__/providers.test.ts b/src/server/__tests__/providers.test.ts
index e5fd792b..82235c58 100644
--- a/src/server/__tests__/providers.test.ts
+++ b/src/server/__tests__/providers.test.ts
@@ -2415,6 +2415,76 @@ describe('ProviderService', () => {
   })
 
   describe('testProviderConfig', () => {
+    for (const [basePath, messagePath] of [
+      ['', '/v1/messages'],
+      ['/', '/v1/messages'],
+      ['/v1', '/v1/messages'],
+      ['/v1///', '/v1/messages'],
+      ['/anthropic', '/anthropic/v1/messages'],
+      ['/anthropic/v1/', '/anthropic/v1/messages'],
+      ['/v1/tenant', '/v1/tenant/v1/messages'],
+    ]) {
+      test(`Anthropic base URL ${basePath || '(root)'} works in connectivity and proxy requests (#1279)`, async () => {
+        const requests: Array<{ path: string; apiKey: string | null; version: string | null }> = []
+        const server = Bun.serve({
+          hostname: '127.0.0.1',
+          port: 0,
+          async fetch(req) {
+            const requestPath = new URL(req.url).pathname
+            requests.push({
+              path: requestPath,
+              apiKey: req.headers.get('x-api-key'),
+              version: req.headers.get('anthropic-version'),
+            })
+            if (requestPath !== messagePath) {
+              return Response.json({ error: { message: 'Unknown endpoint' } }, { status: 404 })
+            }
+            const body = await req.json() as { stream?: boolean }
+            if (body.stream) {
+              return new Response('event: message_stop\ndata: {"type":"message_stop"}\n\n', {
+                headers: { 'Content-Type': 'text/event-stream' },
+              })
+            }
+            return Response.json({ type: 'message', model: 'model-main', content: [{ type: 'text', text: 'ok' }] })
+          },
+        })
+
+        try {
+          const svc = new ProviderService()
+          const baseUrl = `http://127.0.0.1:${server.port}${basePath}`
+          const provider = await svc.addProvider(sampleInput({
+            baseUrl,
+            authStrategy: 'api_key',
+            supportsNestedToolResultMedia: false,
+          }))
+          const result = await svc.testProvider(provider.id)
+          expect(requests[0]?.path).toBe(messagePath)
+          expect(result.connectivity.success).toBe(true)
+          expect(result.proxy?.success).toBe(true)
+
+          for (const stream of [false, true]) {
+            const req = new Request(`http://localhost/proxy/providers/${provider.id}/v1/messages`, {
+              method: 'POST',
+              headers: { 'Content-Type': 'application/json', 'anthropic-version': '2023-06-01' },
+              body: JSON.stringify({ model: 'model-main', max_tokens: 16, stream, messages: [{ role: 'user', content: 'hello' }] }),
+            })
+            const response = await handleProxyRequest(req, new URL(req.url))
+            expect(response.status).toBe(200)
+            if (stream) expect(await response.text()).toContain('event: message_stop')
+            else expect(await response.json()).toMatchObject({ type: 'message' })
+          }
+          expect(requests).toEqual(Array.from({ length: 4 }, () => ({
+            path: messagePath,
+            apiKey: 'sk-test-key-123',
+            version: '2023-06-01',
+          })))
+          expect((await svc.getProvider(provider.id))?.baseUrl).toBe(baseUrl)
+        } finally {
+          server.stop(true)
+        }
+      })
+    }
+
     test('should use auth strategy headers for Anthropic-compatible tests', async () => {
       const originalFetch = globalThis.fetch
       const calls: Array<{ url: string; headers: Record<string, string> }> = []
diff --git a/src/server/__tests__/title-service.test.ts b/src/server/__tests__/title-service.test.ts
index ca6483e7..9819e0fe 100644
--- a/src/server/__tests__/title-service.test.ts
+++ b/src/server/__tests__/title-service.test.ts
@@ -37,6 +37,33 @@ describe('titleService', () => {
     await fs.rm(tmpDir, { recursive: true, force: true })
   })
 
+  test('generates titles with a versioned Anthropic base URL (#1279)', async () => {
+    const paths: string[] = []
+    const server = Bun.serve({
+      hostname: '127.0.0.1',
+      port: 0,
+      fetch(req) {
+        const requestPath = new URL(req.url).pathname
+        paths.push(requestPath)
+        if (requestPath !== '/anthropic/v1/messages') {
+          return Response.json({ error: 'Unknown endpoint' }, { status: 404 })
+        }
+        return Response.json({ content: [{ type: 'text', text: '{"title":"Endpoint compatibility"}' }] })
+      },
+    })
+    try {
+      const provider = await new ProviderService().addProvider({
+        presetId: 'custom', name: 'Versioned Anthropic', apiKey: 'test-key',
+        baseUrl: `http://127.0.0.1:${server.port}/anthropic/v1/`, apiFormat: 'anthropic',
+        models: { main: 'test-model', haiku: 'test-model', sonnet: 'test-model', opus: 'test-model' },
+      })
+      expect(await generateTitle('Explain endpoint compatibility', provider.id)).toBe('Endpoint compatibility')
+      expect(paths).toEqual(['/anthropic/v1/messages'])
+    } finally {
+      server.stop(true)
+    }
+  })
+
   test('sends disabled thinking for title generation by default', async () => {
     let requestBody: Record<string, unknown> | null = null
     const server = Bun.serve({
diff --git a/src/server/proxy/handler.ts b/src/server/proxy/handler.ts
index aa7c8844..5ad090c2 100644
--- a/src/server/proxy/handler.ts
+++ b/src/server/proxy/handler.ts
@@ -10,6 +10,7 @@
  */
 
 import { getOpenAIPolicyError } from '../../services/openaiAuth/policyError.js'
+import { normalizeAnthropicBaseUrl } from '../../services/api/anthropicBaseUrl.js'
 import { createGunzip, createInflate } from 'node:zlib'
 
 import { ProviderService } from '../services/providerService.js'
@@ -442,7 +443,7 @@ async function handleAnthropicCompatible(
   traceContext: ProxyTraceContext | null,
 ): Promise<Response> {
   const transformed = hoistToolResultMediaForCompatibility(body)
-  const url = `${baseUrl}/v1/messages`
+  const url = `${normalizeAnthropicBaseUrl(baseUrl)}/v1/messages`
   const proxyOptions = getNetworkProxyFetchOptions(networkSettings, url)
 
   const headers: Record<string, string> = {
diff --git a/src/server/services/providerService.ts b/src/server/services/providerService.ts
index 7a5ab91c..703f07d2 100644
--- a/src/server/services/providerService.ts
+++ b/src/server/services/providerService.ts
@@ -10,6 +10,7 @@ import * as fs from 'fs/promises'
 import * as path from 'path'
 import * as os from 'os'
 import { ApiError } from '../middleware/errorHandler.js'
+import { normalizeAnthropicBaseUrl } from '../../services/api/anthropicBaseUrl.js'
 import { readRecoverableJsonFile } from './recoverableJsonFile.js'
 import { ManagedSettingsService } from './managedSettingsService.js'
 import { anthropicToOpenaiChat } from '../proxy/transform/anthropicToOpenaiChat.js'
@@ -757,7 +758,7 @@ export class ProviderService {
         headers = { 'Content-Type': 'application/json', Authorization: `Bearer ${apiKey}` }
       } else {
         transformedBody = hoistToolResultMediaForCompatibility(anthropicReq)
-        upstreamUrl = `${base}/v1/messages`
+        upstreamUrl = `${normalizeAnthropicBaseUrl(base)}/v1/messages`
         headers = {
           'Content-Type': 'application/json',
           'anthropic-version': '2023-06-01',
@@ -836,7 +837,7 @@ function buildDirectTestRequest(
   }
   // anthropic
   return {
-    url: `${base}/v1/messages`,
+    url: `${normalizeAnthropicBaseUrl(base)}/v1/messages`,
     headers: {
       'Content-Type': 'application/json',
       'anthropic-version': '2023-06-01',
diff --git a/src/server/services/titleService.ts b/src/server/services/titleService.ts
index 4f9dbe02..1413676d 100644
--- a/src/server/services/titleService.ts
+++ b/src/server/services/titleService.ts
@@ -7,6 +7,7 @@
  */
 
 import { ProviderService } from './providerService.js'
+import { normalizeAnthropicBaseUrl } from '../../services/api/anthropicBaseUrl.js'
 import { getPresetAuthStrategy } from './providerRuntimeEnv.js'
 import {
   getNetworkProxyFetchOptions,
@@ -195,7 +196,7 @@ export async function generateTitle(
     if (!resolvedProvider?.baseUrl || !resolvedProvider?.apiKey) return null
 
     const model = resolvedProvider.models.haiku || resolvedProvider.models.main
-    const url = `${resolvedProvider.baseUrl.replace(/\/+$/, '')}/v1/messages`
+    const url = `${normalizeAnthropicBaseUrl(resolvedProvider.baseUrl.replace(/\/+$/, ''))}/v1/messages`
     const authStrategy = resolvedProvider.authStrategy ?? getPresetAuthStrategy(resolvedProvider.presetId)
     const requestHeaders = buildAnthropicTitleRequestHeaders(resolvedProvider.apiKey, authStrategy)
     const requestBody = {
diff --git a/src/services/api/anthropicBaseUrl.test.ts b/src/services/api/anthropicBaseUrl.test.ts
new file mode 100644
index 00000000..e44e516e
--- /dev/null
+++ b/src/services/api/anthropicBaseUrl.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, test } from 'bun:test'
+import { normalizeAnthropicBaseUrl } from './anthropicBaseUrl.js'
+
+describe('normalizeAnthropicBaseUrl', () => {
+  test.each([
+    ['https://gateway.example', 'https://gateway.example'],
+    ['https://gateway.example/', 'https://gateway.example/'],
+    ['https://gateway.example/v1', 'https://gateway.example'],
+    ['https://gateway.example/v1///', 'https://gateway.example'],
+    ['http://127.0.0.1:1234/anthropic/v1/', 'http://127.0.0.1:1234/anthropic'],
+    ['https://gateway.example/a%2Fb/v1', 'https://gateway.example/a%2Fb'],
+    ['https://gateway.example/v1/tenant', 'https://gateway.example/v1/tenant'],
+    ['https://gateway.example/v10', 'https://gateway.example/v10'],
+    ['https://gateway.example/v1/messages', 'https://gateway.example/v1/messages'],
+    ['https://gateway.example/?route=/v1', 'https://gateway.example/?route=/v1'],
+    ['https://gateway.example/v1?route=one', 'https://gateway.example/v1?route=one'],
+    ['https://gateway.example/#/v1', 'https://gateway.example/#/v1'],
+    ['invalid/v1', 'invalid/v1'],
+    ['file:///v1', 'file:///v1'],
+    ['', ''],
+  ])('%s → %s', (input, expected) => {
+    expect(normalizeAnthropicBaseUrl(input)).toBe(expected)
+  })
+})
diff --git a/src/services/api/anthropicBaseUrl.ts b/src/services/api/anthropicBaseUrl.ts
new file mode 100644
index 00000000..77a48608
--- /dev/null
+++ b/src/services/api/anthropicBaseUrl.ts
@@ -0,0 +1,23 @@
+/**
+ * Anthropic SDK and HTTP callers append /v1 themselves. Accept bases ending in
+ * /v1 without duplicating that final segment; keep gateway prefixes intact.
+ * Normalize only at the request boundary, never in persisted provider settings.
+ */
+export function normalizeAnthropicBaseUrl(baseUrl: string): string {
+  try {
+    const url = new URL(baseUrl)
+    // Query/fragment bases are not supported by the SDK's string concatenation.
+    // Leave them unchanged rather than silently rewriting routing parameters.
+    if (
+      (url.protocol !== 'http:' && url.protocol !== 'https:') ||
+      url.search || url.hash ||
+      !url.pathname.replace(/\/+$/, '').endsWith('/v1')
+    ) return baseUrl
+
+    // Preserve the original spelling/encoding of the gateway prefix.
+    return baseUrl.replace(/\/v1\/*$/, '')
+  } catch {
+    // Keep invalid-input handling at the existing SDK/fetch boundary.
+    return baseUrl
+  }
+}
diff --git a/src/services/api/client.test.ts b/src/services/api/client.test.ts
index 5f14cee1..d7effb6c 100644
--- a/src/services/api/client.test.ts
+++ b/src/services/api/client.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, mock, test } from 'bun:test'
 import * as fs from 'node:fs/promises'
 import * as os from 'node:os'
 import * as path from 'node:path'
+import { createSandboxedTestEnvironment } from '../../../scripts/pr/test-environment.js'
 import { GROK_OAUTH_DUMMY_KEY } from '../grokAuth/fetch.js'
 
 mock.module('src/utils/http.js', () => ({
@@ -133,6 +134,74 @@ describe('shouldUseOpenAICodexTransport', () => {
 })
 
 describe('getAnthropicClient', () => {
+  test('uses one API version in direct message and token-count requests while preserving path prefixes', async () => {
+    const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), 'anthropic-client-base-url-'))
+    const previousEnv = { ...process.env }
+    const testEnv = createSandboxedTestEnvironment(tempDir, {
+      ANTHROPIC_API_KEY: 'sk-test-provider',
+      CLAUDE_CODE_SIMPLE: '1',
+      OPENAI_CODEX_OAUTH_FILE: path.join(tempDir, 'missing-openai-oauth.json'),
+    })
+    for (const key of Object.keys(process.env)) delete process.env[key]
+    Object.assign(process.env, testEnv)
+
+    try {
+      const { getAnthropicClient } = await import('./client.js')
+      for (const [basePath, expectedPrefix] of [
+        ['', ''],
+        ['/', ''],
+        ['/v1', ''],
+        ['/v1/', ''],
+        ['/anthropic', '/anthropic'],
+        ['/anthropic/v1/', '/anthropic'],
+        ['/v1/tenant', '/v1/tenant'],
+        ['/proxy/providers/provider-1', '/proxy/providers/provider-1'],
+      ]) {
+        process.env.ANTHROPIC_BASE_URL = `https://provider.example${basePath}`
+        const receivedUrls: string[] = []
+        const client = await getAnthropicClient({
+          apiKey: 'sk-test-provider',
+          maxRetries: 0,
+          model: 'claude-sonnet-4-6',
+          fetchOverride: async input => {
+            const url = new URL(input instanceof Request ? input.url : String(input))
+            receivedUrls.push(url.href)
+            return url.pathname.endsWith('/count_tokens')
+              ? Response.json({ input_tokens: 3 })
+              : Response.json({
+                  id: 'msg-test-base-url',
+                  type: 'message',
+                  role: 'assistant',
+                  model: 'claude-sonnet-4-6',
+                  content: [{ type: 'text', text: 'ok' }],
+                  stop_reason: 'end_turn',
+                  stop_sequence: null,
+                  usage: { input_tokens: 3, output_tokens: 1 },
+                })
+          },
+        })
+        const messages = [{ role: 'user' as const, content: 'hello' }]
+        const response = await client.messages.create({
+          model: 'claude-sonnet-4-6',
+          max_tokens: 16,
+          messages,
+        })
+        const count = await client.beta.messages.countTokens({ model: 'claude-sonnet-4-6', messages })
+
+        expect(response.id).toBe('msg-test-base-url')
+        expect(count.input_tokens).toBe(3)
+        expect(receivedUrls).toEqual([
+          `https://provider.example${expectedPrefix}/v1/messages`,
+          `https://provider.example${expectedPrefix}/v1/messages/count_tokens?beta=true`,
+        ])
+      }
+    } finally {
+      for (const key of Object.keys(process.env)) delete process.env[key]
+      Object.assign(process.env, previousEnv)
+      await fs.rm(tempDir, { recursive: true, force: true })
+    }
+  })
+
   test('selects the isolated Grok transport with a dummy SDK key', async () => {
     const { getAnthropicClient } = await import('./client.js')
     const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), 'grok-client-test-'))
diff --git a/src/services/api/client.ts b/src/services/api/client.ts
index 0a6d3e0b..b2c7c820 100644
--- a/src/services/api/client.ts
+++ b/src/services/api/client.ts
@@ -1,4 +1,5 @@
 import Anthropic, { type ClientOptions } from '@anthropic-ai/sdk'
+import { normalizeAnthropicBaseUrl } from './anthropicBaseUrl.js'
 import { randomUUID } from 'crypto'
 import type { GoogleAuth } from 'google-auth-library'
 import {
@@ -451,8 +452,8 @@ export async function getAnthropicClient({
     authToken: isClaudeSubscriber && !usingOpenAICodex && !usingGrok
       ? getClaudeAIOAuthTokens()?.accessToken
       : undefined,
-    // Set baseURL from OAuth config when using staging OAuth
-    ...(stagingOAuthBaseUrl ? { baseURL: stagingOAuthBaseUrl } : {}),
+    // The SDK appends /v1 to every API path, including messages and token counts.
+    ...(baseURL ? { baseURL: normalizeAnthropicBaseUrl(baseURL) } : {}),
     ...ARGS,
     ...(isDebugToStdErr() && { logger: createStderrLogger() }),
   }
diff --git a/src/services/api/filesApi.test.ts b/src/services/api/filesApi.test.ts
new file mode 100644
index 00000000..87c6f925
--- /dev/null
+++ b/src/services/api/filesApi.test.ts
@@ -0,0 +1,163 @@
+import { describe, expect, test } from 'bun:test'
+import * as fs from 'node:fs/promises'
+import * as os from 'node:os'
+import * as path from 'node:path'
+import { createSandboxedTestEnvironment } from '../../../scripts/pr/test-environment.js'
+import type { FilesApiConfig } from './filesApi.js'
+
+type RecordedRequest = {
+  method: string
+  url: URL
+  headers: Headers
+  body: string
+}
+
+const basePaths = [
+  ['', ''],
+  ['/v1', ''],
+  ['/v1/', ''],
+  ['/anthropic', '/anthropic'],
+  ['/anthropic/v1/', '/anthropic'],
+  ['/v1/tenant', '/v1/tenant'],
+  ['/proxy/providers/provider-1', '/proxy/providers/provider-1'],
+] as const
+
+async function withFilesApi(
+  run: (fixture: {
+    api: typeof import('./filesApi.js')
+    origin: string
+    requests: RecordedRequest[]
+    uploadPath: string
+    config: FilesApiConfig
+  }) => Promise<void>,
+): Promise<void> {
+  const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), 'files-api-base-url-'))
+  const previousEnv = { ...process.env }
+  const env = createSandboxedTestEnvironment(tempDir)
+  for (const key of Object.keys(process.env)) delete process.env[key]
+  Object.assign(process.env, env)
+  const requests: RecordedRequest[] = []
+  const server = Bun.serve({
+    hostname: '127.0.0.1',
+    port: 0,
+    async fetch(request) {
+      const url = new URL(request.url)
+      requests.push({ method: request.method, url, headers: request.headers, body: await request.text() })
+      if (request.method === 'POST') return Response.json({ id: 'file-uploaded' }, { status: 201 })
+      if (url.pathname.endsWith('/content')) return new Response('downloaded content')
+      const secondPage = url.searchParams.has('after_id')
+      return Response.json({
+        data: [{ id: secondPage ? 'file-2' : 'file-1', filename: secondPage ? 'two.txt' : 'one.txt', size_bytes: 7 }],
+        has_more: !secondPage,
+      })
+    },
+  })
+
+  try {
+    const uploadPath = path.join(tempDir, 'upload.txt')
+    await fs.writeFile(uploadPath, 'upload fixture content')
+    await run({
+      api: await import('./filesApi.js'),
+      origin: server.url.origin,
+      requests,
+      uploadPath,
+      config: { oauthToken: 'fake-files-oauth-token', sessionId: 'test-files-session' },
+    })
+  } finally {
+    server.stop(true)
+    for (const key of Object.keys(process.env)) delete process.env[key]
+    Object.assign(process.env, previousEnv)
+    await fs.rm(tempDir, { recursive: true, force: true })
+  }
+}
+
+function expectFilesAuth(requests: RecordedRequest[]): void {
+  for (const request of requests) {
+    expect(request.headers.get('authorization')).toBe('Bearer fake-files-oauth-token')
+    expect(request.headers.get('anthropic-version')).toBe('2023-06-01')
+    expect(request.headers.get('anthropic-beta')).toBe('files-api-2025-04-14,oauth-2025-04-20')
+  }
+}
+
+describe('Files API base URL compatibility', () => {
+  test('downloads with one API version while preserving gateway prefixes and authentication', async () => {
+    await withFilesApi(async ({ api, origin, requests, config }) => {
+      for (const [basePath, prefix] of basePaths) {
+        requests.length = 0
+        const content = await api.downloadFile('file-1', { ...config, baseUrl: `${origin}${basePath}` })
+
+        expect(content.toString()).toBe('downloaded content')
+        expect(requests.map(request => [request.method, request.url.href])).toEqual([
+          ['GET', `${origin}${prefix}/v1/files/file-1/content`],
+        ])
+        expectFilesAuth(requests)
+      }
+    })
+  })
+
+  test('uploads with one API version and preserves the multipart payload', async () => {
+    await withFilesApi(async ({ api, origin, requests, config, uploadPath }) => {
+      for (const [basePath, prefix] of basePaths) {
+        requests.length = 0
+        const result = await api.uploadFile(uploadPath, 'folder/report.txt', { ...config, baseUrl: `${origin}${basePath}` })
+
+        expect(result).toEqual({ path: 'folder/report.txt', fileId: 'file-uploaded', size: 22, success: true })
+        expect(requests.map(request => [request.method, request.url.href])).toEqual([
+          ['POST', `${origin}${prefix}/v1/files`],
+        ])
+        expect(requests[0]!.headers.get('content-type')).toStartWith('multipart/form-data; boundary=')
+        expect(requests[0]!.body).toContain('filename="report.txt"')
+        expect(requests[0]!.body).toContain('upload fixture content')
+        expect(requests[0]!.body).toContain('name="purpose"\r\n\r\nuser_data')
+        expectFilesAuth(requests)
+      }
+    })
+  })
+
+  test('lists every page with one API version and preserves filter and cursor parameters', async () => {
+    await withFilesApi(async ({ api, origin, requests, config }) => {
+      const afterCreatedAt = '2026-09-08T00:00:00Z'
+      for (const [basePath, prefix] of basePaths) {
+        requests.length = 0
+        const files = await api.listFilesCreatedAfter(afterCreatedAt, { ...config, baseUrl: `${origin}${basePath}` })
+
+        expect(files).toEqual([
+          { filename: 'one.txt', fileId: 'file-1', size: 7 },
+          { filename: 'two.txt', fileId: 'file-2', size: 7 },
+        ])
+        expect(requests.map(request => [request.method, request.url.pathname])).toEqual([
+          ['GET', `${prefix}/v1/files`],
+          ['GET', `${prefix}/v1/files`],
+        ])
+        expect(requests.map(request => Object.fromEntries(request.url.searchParams))).toEqual([
+          { after_created_at: afterCreatedAt },
+          { after_created_at: afterCreatedAt, after_id: 'file-1' },
+        ])
+        expectFilesAuth(requests)
+      }
+    })
+  })
+
+  test('normalizes environment fallback URLs for downloads, uploads, and listing', async () => {
+    await withFilesApi(async ({ api, origin, requests, config, uploadPath }) => {
+      for (const envKey of ['ANTHROPIC_BASE_URL', 'CLAUDE_CODE_API_BASE_URL']) {
+        delete process.env.ANTHROPIC_BASE_URL
+        delete process.env.CLAUDE_CODE_API_BASE_URL
+        process.env[envKey] = `${origin}/gateway/v1/`
+        requests.length = 0
+        await api.downloadFile('file-1', config)
+        await api.uploadFile(uploadPath, 'report.txt', config)
+        await api.listFilesCreatedAfter('2026-09-08T00:00:00Z', config)
+
+        expect(requests.map(request => request.url.pathname)).toEqual([
+          '/gateway/v1/files/file-1/content',
+          '/gateway/v1/files',
+          '/gateway/v1/files',
+          '/gateway/v1/files',
+        ])
+        expect(process.env[envKey]).toBe(`${origin}/gateway/v1/`)
+        expectFilesAuth(requests)
+      }
+    })
+  })
+})
diff --git a/src/services/api/filesApi.ts b/src/services/api/filesApi.ts
index cb9a03b9..b385b669 100644
--- a/src/services/api/filesApi.ts
+++ b/src/services/api/filesApi.ts
@@ -21,6 +21,7 @@ import {
   type AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS,
   logEvent,
 } from '../analytics/index.js'
+import { normalizeAnthropicBaseUrl } from './anthropicBaseUrl.js'
 
 // Files API is currently in beta. oauth-2025-04-20 enables Bearer OAuth
 // on public-api routes (auth.py: "oauth_auth" not in beta_versions → 404).
@@ -133,7 +134,7 @@ export async function downloadFile(
   fileId: string,
   config: FilesApiConfig,
 ): Promise<Buffer> {
-  const baseUrl = config.baseUrl || getDefaultApiBaseUrl()
+  const baseUrl = normalizeAnthropicBaseUrl(config.baseUrl || getDefaultApiBaseUrl())
   const url = `${baseUrl}/v1/files/${fileId}/content`
 
   const headers = {
@@ -381,7 +382,7 @@ export async function uploadFile(
   config: FilesApiConfig,
   opts?: { signal?: AbortSignal },
 ): Promise<UploadResult> {
-  const baseUrl = config.baseUrl || getDefaultApiBaseUrl()
+  const baseUrl = normalizeAnthropicBaseUrl(config.baseUrl || getDefaultApiBaseUrl())
   const url = `${baseUrl}/v1/files`
 
   const headers = {
@@ -618,7 +619,7 @@ export async function listFilesCreatedAfter(
   afterCreatedAt: string,
   config: FilesApiConfig,
 ): Promise<FileMetadata[]> {
-  const baseUrl = config.baseUrl || getDefaultApiBaseUrl()
+  const baseUrl = normalizeAnthropicBaseUrl(config.baseUrl || getDefaultApiBaseUrl())
   const headers = {
     Authorization: `Bearer ${config.oauthToken}`,
     'anthropic-version': ANTHROPIC_VERSION,