diff --git a/CLAUDE.md b/CLAUDE.md
index 3e3386a..2c609e9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -88,6 +88,12 @@ The server adapter in `server.ts` collects these registrations into an internal
 
 `ClientRegistry` (`src/seatable/clientRegistry.ts`) manages multiple `SeaTableClient` instances keyed by base name. `ContextualClient` (`src/seatable/contextualClient.ts`) implements `ClientLike` and proxies calls to the right client based on a `base` parameter. In multi-base mode, `handleCallTool()` extracts the `base` arg and `handleListTools()` injects it into every tool schema dynamically — no changes needed in individual tool files.
 
+### Multi-base and the base context
+
+`ContextualClient` binds the target base to an `AsyncLocalStorage` scope opened by `runWithBase()`, which `handleCallTool()` wraps around the tool handler. **Everything that needs the client must read it inside that scope.** Reading after it returns throws `ContextualClient was used outside runWithBase()` — deliberately, and identically whether one base or ten are configured.
+
+That guard exists because the opposite behaviour hid a bug for five months (fixed in #5): the post-handler log line called `getBaseInfo()` after the scope had closed. With a single base it silently succeeded via the registry default; with two or more `resolve(undefined)` threw `Multiple bases available … Specify "base" parameter`, which reads like a caller mistake and discarded a result the handler had already produced correctly. Coverage is in `tests/contextualClient.spec.ts` and `tests/server.spec.ts`.
+
 ### Schema Utilities
 
 - `src/schema/map.ts` — converts SeaTable metadata to `GenericSchema` format
diff --git a/package.json b/package.json
index 0798628..60ddeeb 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@seatable/mcp-seatable",
-  "version": "1.6.3",
+  "version": "1.6.4",
   "type": "module",
   "license": "MIT",
   "mcpName": "io.github.seatable/seatable",
diff --git a/src/seatable/contextualClient.ts b/src/seatable/contextualClient.ts
index f8bd93d..8f43762 100644
--- a/src/seatable/contextualClient.ts
+++ b/src/seatable/contextualClient.ts
@@ -3,7 +3,12 @@ import { AsyncLocalStorage } from 'node:async_hooks'
 import type { ClientLike } from '../mcp/tools/types.js'
 import type { ClientRegistry } from './clientRegistry.js'
 
-const baseContext = new AsyncLocalStorage<string | undefined>()
+/**
+ * The store is an object rather than the bare name so that "no scope at all"
+ * (getStore() === undefined) stays distinguishable from "in a scope, no base
+ * named" ({ base: undefined }). Those two need very different errors.
+ */
+const baseContext = new AsyncLocalStorage<{ base?: string }>()
 
 /**
  * A ClientLike proxy that delegates to a specific base client
@@ -24,11 +29,26 @@ export class ContextualClient implements ClientLike {
      * will route to the specified base.
      */
     runWithBase<T>(name: string | undefined, fn: () => T): T {
-        return baseContext.run(name, fn)
+        return baseContext.run({ base: name }, fn)
     }
 
+    /**
+     * Resolving outside a runWithBase() scope is always a programming error, and
+     * it used to hide well: with a single base it quietly succeeded via the
+     * registry default, and only with two or more did it surface — as "Specify
+     * base parameter", which blames the caller for omitting an argument they
+     * did supply. It cost five months to find that way once, so it is named
+     * here and fails the same way whatever the base count.
+     */
     private get client(): ClientLike {
-        return this.registry.resolve(baseContext.getStore())
+        const store = baseContext.getStore()
+        if (!store) {
+            throw new Error(
+                'ContextualClient was used outside runWithBase(). The base context only exists for the '
+                + 'duration of that scope — read what you need inside it rather than after it returns.'
+            )
+        }
+        return this.registry.resolve(store.base)
     }
 
     // Base info
diff --git a/tests/contextualClient.spec.ts b/tests/contextualClient.spec.ts
index 5639292..fce57bc 100644
--- a/tests/contextualClient.spec.ts
+++ b/tests/contextualClient.spec.ts
@@ -74,6 +74,37 @@ describe('ContextualClient', () => {
         expect(() => ctx.runWithBase(undefined, () => ctx.listTables())).toThrow('Specify "base" parameter')
     })
 
+    /*
+     * The failure mode this guards against cost five months to find: reading the
+     * client after the runWithBase() scope returned. With one base it silently
+     * worked (resolve(undefined) fell back to the default), with two it failed as
+     * "Specify base parameter" — which reads like the caller forgot an argument
+     * they had in fact supplied. Both halves are wrong, so the escape is named.
+     */
+    it('throws a diagnostic error when used outside runWithBase()', () => {
+        const registry = createMockRegistry(['A', 'B'])
+        const ctx = new ContextualClient(registry as any)
+
+        expect(() => ctx.getBaseInfo()).toThrow(/outside runWithBase/)
+    })
+
+    it('throws the same diagnostic outside the scope even with a single base', () => {
+        // The single-base case used to succeed here, which is exactly what kept
+        // the defect invisible until someone configured a second base.
+        const registry = createMockRegistry(['OnlyBase'])
+        const ctx = new ContextualClient(registry as any)
+
+        expect(() => ctx.listTables()).toThrow(/outside runWithBase/)
+    })
+
+    it('still reports a genuinely missing base argument inside the scope', () => {
+        const registry = createMockRegistry(['A', 'B'])
+        const ctx = new ContextualClient(registry as any)
+
+        // Inside the scope with no name, the caller really did omit "base".
+        expect(() => ctx.runWithBase(undefined, () => ctx.listTables())).toThrow('Specify "base" parameter')
+    })
+
     it('throws for unknown base name', () => {
         const registry = createMockRegistry(['CRM'])
         const ctx = new ContextualClient(registry as any)