diff --git a/.changeset/sandbox-no-workspace-provides.md b/.changeset/sandbox-no-workspace-provides.md
new file mode 100644
index 0000000000..eb2cc8b7da
--- /dev/null
+++ b/.changeset/sandbox-no-workspace-provides.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-sandbox': patch
+---
+
+`withSandbox` no longer fails the first `chat()` call with `declares it provides "sandbox-projection" but never called provide() in setup()` when the sandbox definition has no `workspace`. `sandbox-projection` is listed in `provides` only when a workspace is declared, which is when `setup` calls `provide()`. A sandbox with a workspace still satisfies `requires: [ProjectionCapability]`.
diff --git a/packages/ai-sandbox/src/middleware.ts b/packages/ai-sandbox/src/middleware.ts
index 5c5e951a63..052e00d95a 100644
--- a/packages/ai-sandbox/src/middleware.ts
+++ b/packages/ai-sandbox/src/middleware.ts
@@ -496,17 +496,18 @@ export function withSandbox<TOffset extends string = string>(
 ): DefinedChatMiddleware<
   unknown,
   readonly [],
-  readonly [typeof SandboxCapability, typeof ProjectionCapability]
+  | readonly [typeof SandboxCapability, typeof ProjectionCapability]
+  | readonly [typeof SandboxCapability]
 > {
   return defineChatMiddleware({
     name: 'sandbox',
-    provides: [SandboxCapability, ProjectionCapability],
-    // SandboxPolicyCapability is provided conditionally (only when the
-    // definition has a policy), so it is intentionally NOT declared here —
-    // consumers read it via `getOptional`. SandboxDurabilityCapability and
-    // DetachableRunCapability are conditional for the same reason (only when
-    // `runs` + `durability` are both wired), so they are intentionally NOT
-    // declared here either.
+    // Listed only when setup will call provide(). chat() checks `requires`
+    // against this array before setup. Policy, durability, and detach stay
+    // off the list: they depend on other options and are read optionally.
+    provides:
+      definition.workspace !== undefined
+        ? [SandboxCapability, ProjectionCapability]
+        : [SandboxCapability],
     optionalRequires: [SandboxInstanceStoreCapability, LocksCapability],
 
     async setup(ctx) {
diff --git a/packages/ai-sandbox/tests/tool-capability.test.ts b/packages/ai-sandbox/tests/tool-capability.test.ts
index 9667d51404..d5e9b3c009 100644
--- a/packages/ai-sandbox/tests/tool-capability.test.ts
+++ b/packages/ai-sandbox/tests/tool-capability.test.ts
@@ -1,7 +1,13 @@
 import { describe, expect, it } from 'vitest'
-import { EventType, chat, toolDefinition } from '@tanstack/ai'
+import {
+  EventType,
+  chat,
+  defineChatMiddleware,
+  toolDefinition,
+} from '@tanstack/ai'
 import {
   InMemorySandboxInstanceStore,
+  ProjectionCapability,
   SandboxCapability,
   defineSandbox,
   defineWorkspace,
@@ -79,12 +85,54 @@ describe('withSandbox + a server tool', () => {
       fileEvents: false,
     })
     let seen: SandboxHandle | undefined
+    let projected: boolean | undefined
+    const whereAmI = toolDefinition({
+      name: 'whereAmI',
+      description: 'Return the sandbox id',
+      inputSchema: { type: 'object', properties: {} },
+    }).server((_args, context) => {
+      seen = context?.get?.(SandboxCapability)
+      projected = context?.getOptional?.(ProjectionCapability) !== undefined
+      return seen?.id ?? 'none'
+    })
+
+    const stream = chat({
+      adapter: toolThenAnswer(),
+      messages: [{ role: 'user', content: 'hi' }],
+      runId: 'run-1',
+      threadId: 'thread-1',
+      tools: [whereAmI],
+      middleware: [
+        withSandbox(sandbox, { instances: new InMemorySandboxInstanceStore() }),
+        defineChatMiddleware({
+          name: 'needs-projection',
+          requires: [ProjectionCapability],
+        }),
+      ],
+    })
+    for await (const _chunk of stream) void _chunk
+
+    expect(provider.created).toHaveLength(1)
+    expect(seen).toBe(provider.created[0])
+    expect(projected).toBe(true)
+  })
+
+  it('runs setup when the definition has no workspace (no projection provided)', async () => {
+    const provider = makeFakeProvider({ caps: FULL_CAPS })
+    const sandbox = defineSandbox({
+      id: 'repo',
+      provider,
+      fileEvents: false,
+    })
+    let seen: SandboxHandle | undefined
+    let projected: boolean | undefined
     const whereAmI = toolDefinition({
       name: 'whereAmI',
       description: 'Return the sandbox id',
       inputSchema: { type: 'object', properties: {} },
     }).server((_args, context) => {
       seen = context?.get?.(SandboxCapability)
+      projected = context?.getOptional?.(ProjectionCapability) !== undefined
       return seen?.id ?? 'none'
     })
 
@@ -102,5 +150,6 @@ describe('withSandbox + a server tool', () => {
 
     expect(provider.created).toHaveLength(1)
     expect(seen).toBe(provider.created[0])
+    expect(projected).toBe(false)
   })
 })