diff --git a/crates/bashkit-js/__test__/ai-adapters.spec.ts b/crates/bashkit-js/__test__/ai-adapters.spec.ts
new file mode 100644
index 00000000..304a5c8c
--- /dev/null
+++ b/crates/bashkit-js/__test__/ai-adapters.spec.ts
@@ -0,0 +1,110 @@
+import test from "ava";
+import { bashTool as aiSdkBashTool } from "../ai.js";
+import { bashTool as anthropicBashTool } from "../anthropic.js";
+import { bashTool as openAiBashTool } from "../openai.js";
+
+// ============================================================================
+// Issue #990: AI adapters must use a single interpreter instance.
+// Files written via the exposed `bash` handle must be visible to tool
+// execution, and vice versa — no state divergence.
+// ============================================================================
+
+// --- Vercel AI SDK adapter ---------------------------------------------------
+
+test("ai: files option is readable via execute", async (t) => {
+  const adapter = aiSdkBashTool({ files: { "/data.txt": "hello" } });
+  const result = await adapter.tools.bash.execute({ commands: "cat /data.txt" });
+  t.is(result, "hello");
+});
+
+test("ai: files written via bash.writeFile are visible in execute", async (t) => {
+  const adapter = aiSdkBashTool();
+  adapter.bash.writeFile("/x.txt", "from-api");
+  const result = await adapter.tools.bash.execute({ commands: "cat /x.txt" });
+  t.is(result, "from-api");
+});
+
+test("ai: files created via execute are readable via bash.readFile", async (t) => {
+  const adapter = aiSdkBashTool();
+  await adapter.tools.bash.execute({ commands: "echo -n created > /y.txt" });
+  t.is(adapter.bash.readFile("/y.txt"), "created");
+});
+
+// --- Anthropic SDK adapter ---------------------------------------------------
+
+test("anthropic: files option is readable via handler", async (t) => {
+  const adapter = anthropicBashTool({ files: { "/data.txt": "hello" } });
+  const result = await adapter.handler({
+    type: "tool_use",
+    id: "t1",
+    name: "bash",
+    input: { commands: "cat /data.txt" },
+  });
+  t.is(result.content, "hello");
+  t.false(result.is_error);
+});
+
+test("anthropic: files written via bash.writeFile are visible in handler", async (t) => {
+  const adapter = anthropicBashTool();
+  adapter.bash.writeFile("/x.txt", "from-api");
+  const result = await adapter.handler({
+    type: "tool_use",
+    id: "t2",
+    name: "bash",
+    input: { commands: "cat /x.txt" },
+  });
+  t.is(result.content, "from-api");
+});
+
+test("anthropic: files created via handler are readable via bash.readFile", async (t) => {
+  const adapter = anthropicBashTool();
+  await adapter.handler({
+    type: "tool_use",
+    id: "t3",
+    name: "bash",
+    input: { commands: "echo -n created > /y.txt" },
+  });
+  t.is(adapter.bash.readFile("/y.txt"), "created");
+});
+
+// --- OpenAI SDK adapter ------------------------------------------------------
+
+test("openai: files option is readable via handler", async (t) => {
+  const adapter = openAiBashTool({ files: { "/data.txt": "hello" } });
+  const result = await adapter.handler({
+    id: "c1",
+    type: "function",
+    function: {
+      name: "bash",
+      arguments: JSON.stringify({ commands: "cat /data.txt" }),
+    },
+  });
+  t.is(result.content, "hello");
+});
+
+test("openai: files written via bash.writeFile are visible in handler", async (t) => {
+  const adapter = openAiBashTool();
+  adapter.bash.writeFile("/x.txt", "from-api");
+  const result = await adapter.handler({
+    id: "c2",
+    type: "function",
+    function: {
+      name: "bash",
+      arguments: JSON.stringify({ commands: "cat /x.txt" }),
+    },
+  });
+  t.is(result.content, "from-api");
+});
+
+test("openai: files created via handler are readable via bash.readFile", async (t) => {
+  const adapter = openAiBashTool();
+  await adapter.handler({
+    id: "c3",
+    type: "function",
+    function: {
+      name: "bash",
+      arguments: JSON.stringify({ commands: "echo -n created > /y.txt" }),
+    },
+  });
+  t.is(adapter.bash.readFile("/y.txt"), "created");
+});
diff --git a/crates/bashkit-js/ai.ts b/crates/bashkit-js/ai.ts
index 38191cfb..b2ef4923 100644
--- a/crates/bashkit-js/ai.ts
+++ b/crates/bashkit-js/ai.ts
@@ -27,7 +27,7 @@
  * @packageDocumentation
  */
 
-import { Bash, BashTool } from "./wrapper.js";
+import { BashTool } from "./wrapper.js";
 import type { BashOptions, ExecResult } from "./wrapper.js";
 
 // Vercel AI SDK tool types — we define them inline to avoid requiring
@@ -60,8 +60,8 @@ export interface BashToolAdapter {
   system: string;
   /** Tool definitions for Vercel AI SDK's generateText/streamText. */
   tools: Record<string, AiTool>;
-  /** The underlying Bash instance for direct access. */
-  bash: Bash;
+  /** The underlying BashTool instance for direct access. */
+  bash: BashTool;
 }
 
 function formatOutput(result: ExecResult): string {
@@ -104,8 +104,7 @@ function formatOutput(result: ExecResult): string {
 export function bashTool(options?: BashToolOptions): BashToolAdapter {
   const { files, ...bashOptions } = options ?? {};
 
-  const bashToolInstance = new BashTool(bashOptions);
-  const bash = new Bash(bashOptions);
+  const bash = new BashTool(bashOptions);
 
   if (files) {
     for (const [path, content] of Object.entries(files)) {
@@ -113,11 +112,11 @@ export function bashTool(options?: BashToolOptions): BashToolAdapter {
     }
   }
 
-  const system = bashToolInstance.systemPrompt();
+  const system = bash.systemPrompt();
 
   const tools: Record<string, AiTool> = {
     bash: {
-      description: bashToolInstance.description(),
+      description: bash.description(),
       parameters: {
         type: "object",
         properties: {
diff --git a/crates/bashkit-js/anthropic.ts b/crates/bashkit-js/anthropic.ts
index ab5d3f7b..cbde1294 100644
--- a/crates/bashkit-js/anthropic.ts
+++ b/crates/bashkit-js/anthropic.ts
@@ -31,7 +31,7 @@
  * @packageDocumentation
  */
 
-import { Bash, BashTool } from "./wrapper.js";
+import { BashTool } from "./wrapper.js";
 import type { BashOptions, ExecResult } from "./wrapper.js";
 
 /** Options for configuring the bash tool adapter. */
@@ -75,8 +75,8 @@ export interface BashToolAdapter {
   tools: AnthropicTool[];
   /** Handler that executes a tool_use block and returns a tool_result. */
   handler: (toolUse: ToolUseBlock) => Promise<ToolResult>;
-  /** The underlying Bash instance for direct access. */
-  bash: Bash;
+  /** The underlying BashTool instance for direct access. */
+  bash: BashTool;
 }
 
 function formatOutput(result: ExecResult): string {
@@ -118,8 +118,7 @@ function formatOutput(result: ExecResult): string {
 export function bashTool(options?: BashToolOptions): BashToolAdapter {
   const { files, ...bashOptions } = options ?? {};
 
-  const bashToolInstance = new BashTool(bashOptions);
-  const bash = new Bash(bashOptions);
+  const bash = new BashTool(bashOptions);
 
   // Pre-populate VFS files
   if (files) {
@@ -128,12 +127,12 @@ export function bashTool(options?: BashToolOptions): BashToolAdapter {
     }
   }
 
-  const system = bashToolInstance.systemPrompt();
+  const system = bash.systemPrompt();
 
   const tools: AnthropicTool[] = [
     {
       name: "bash",
-      description: bashToolInstance.description(),
+      description: bash.description(),
       input_schema: {
         type: "object",
         properties: {
diff --git a/crates/bashkit-js/openai.ts b/crates/bashkit-js/openai.ts
index 02741045..ef375451 100644
--- a/crates/bashkit-js/openai.ts
+++ b/crates/bashkit-js/openai.ts
@@ -30,7 +30,7 @@
  * @packageDocumentation
  */
 
-import { Bash, BashTool } from "./wrapper.js";
+import { BashTool } from "./wrapper.js";
 import type { BashOptions, ExecResult } from "./wrapper.js";
 
 /** Options for configuring the bash tool adapter. */
@@ -78,8 +78,8 @@ export interface BashToolAdapter {
   tools: OpenAITool[];
   /** Handler that executes a tool_call and returns a tool message. */
   handler: (toolCall: OpenAIToolCall) => Promise<ToolResult>;
-  /** The underlying Bash instance for direct access. */
-  bash: Bash;
+  /** The underlying BashTool instance for direct access. */
+  bash: BashTool;
 }
 
 function formatOutput(result: ExecResult): string {
@@ -122,8 +122,7 @@ function formatOutput(result: ExecResult): string {
 export function bashTool(options?: BashToolOptions): BashToolAdapter {
   const { files, ...bashOptions } = options ?? {};
 
-  const bashToolInstance = new BashTool(bashOptions);
-  const bash = new Bash(bashOptions);
+  const bash = new BashTool(bashOptions);
 
   if (files) {
     for (const [path, content] of Object.entries(files)) {
@@ -131,14 +130,14 @@ export function bashTool(options?: BashToolOptions): BashToolAdapter {
     }
   }
 
-  const system = bashToolInstance.systemPrompt();
+  const system = bash.systemPrompt();
 
   const tools: OpenAITool[] = [
     {
       type: "function",
       function: {
         name: "bash",
-        description: bashToolInstance.description(),
+        description: bash.description(),
         parameters: {
           type: "object",
           properties: {