diff --git a/src/lib/steps/harness/anthropic.ts b/src/lib/steps/harness/anthropic.ts
index 54c5f85..bbed7ba 100644
--- a/src/lib/steps/harness/anthropic.ts
+++ b/src/lib/steps/harness/anthropic.ts
@@ -1,5 +1,6 @@
-import { query } from '@anthropic-ai/claude-agent-sdk'
+import { type HookCallback, query } from '@anthropic-ai/claude-agent-sdk'
 
+import { toolInputTouchesSecret } from './secret-paths.js'
 import type { Harness, HarnessRunStepArgs, StepRunResult } from './types.js'
 
 // The official Seam MCP (same server the seam-plugin wires up).
@@ -7,17 +8,38 @@ const SEAM_MCP_URL = 'https://mcp.seam.co/mcp'
 
 // Read/search/write + the docs MCP. Deliberately no Bash, no subagents, no task
 // tools: the agent writes integration code, it does not run the developer's
-// shell. `mcp__seam-docs__*` grants every seam-docs tool.
+// shell. No WebFetch either — the agent gets its references from the seam-docs
+// MCP, so arbitrary web egress is unnecessary and only widens the exfiltration
+// surface. `mcp__seam-docs__*` grants every seam-docs tool.
 const ALLOWED_TOOLS = [
   'Read',
   'Glob',
   'Grep',
   'Edit',
   'Write',
-  'WebFetch',
   'mcp__seam-docs__*',
 ]
 
+// Hard block on the developer's secret files: a deny here overrides the broad
+// `Read` allow above and fires for every tool, so Read/Grep/Edit/Write can't
+// touch `.env` (the system-prompt instruction alone is not enforcement).
+const denySecretFileAccess: HookCallback = async (input) => {
+  if (
+    input.hook_event_name === 'PreToolUse' &&
+    toolInputTouchesSecret(input.tool_input)
+  ) {
+    return {
+      hookSpecificOutput: {
+        hookEventName: 'PreToolUse',
+        permissionDecision: 'deny',
+        permissionDecisionReason:
+          'Reading .env / secret files is blocked. Load SEAM_API_KEY from the runtime environment instead.',
+      },
+    }
+  }
+  return {}
+}
+
 // The control harness: drives the integration with the Claude Agent SDK.
 export const anthropicHarness: Harness = {
   name: 'anthropic',
@@ -54,6 +76,10 @@ export const anthropicHarness: Harness = {
         // reviews the result as a git diff afterward. Read/search tools and the
         // docs MCP are read-only, so nothing destructive runs unattended.
         permissionMode: 'acceptEdits',
+        // Block reads/writes of the developer's .env / secret files.
+        hooks: {
+          PreToolUse: [{ hooks: [denySecretFileAccess] }],
+        },
         mcpServers: {
           // Wired exactly like the seam-plugin: mcp-remote bridges to the hosted
           // Seam MCP and runs the OAuth browser flow on first use, caching the
diff --git a/src/lib/steps/harness/secret-paths.test.ts b/src/lib/steps/harness/secret-paths.test.ts
new file mode 100644
index 0000000..6af12bd
--- /dev/null
+++ b/src/lib/steps/harness/secret-paths.test.ts
@@ -0,0 +1,40 @@
+import { expect, test } from 'vitest'
+
+import { isSecretFilePath, toolInputTouchesSecret } from './secret-paths.js'
+
+test('isSecretFilePath: blocks .env and its variants', () => {
+  expect(isSecretFilePath('.env')).toBe(true)
+  expect(isSecretFilePath('.env.local')).toBe(true)
+  expect(isSecretFilePath('.env.production')).toBe(true)
+  expect(isSecretFilePath('./.env')).toBe(true)
+  expect(isSecretFilePath('/abs/project/.env')).toBe(true)
+  expect(isSecretFilePath('config/.env.staging')).toBe(true)
+})
+
+test('isSecretFilePath: allows the value-less .env.example template', () => {
+  expect(isSecretFilePath('.env.example')).toBe(false)
+  expect(isSecretFilePath('./.env.example')).toBe(false)
+  expect(isSecretFilePath('nested/dir/.env.example')).toBe(false)
+})
+
+test('isSecretFilePath: allows ordinary files', () => {
+  expect(isSecretFilePath('src/index.ts')).toBe(false)
+  expect(isSecretFilePath('README.md')).toBe(false)
+  // Not a dotenv file — "environment.ts" merely starts with "env" after a slash.
+  expect(isSecretFilePath('src/environment.ts')).toBe(false)
+})
+
+test('toolInputTouchesSecret: detects the path across tool input fields', () => {
+  expect(toolInputTouchesSecret({ file_path: '.env' })).toBe(true) // Read/Edit/Write
+  expect(toolInputTouchesSecret({ path: './.env.local' })).toBe(true) // Grep/Glob/Ls
+  expect(toolInputTouchesSecret({ glob: '.env' })).toBe(true) // Grep glob
+  expect(toolInputTouchesSecret({ file_path: '.env.example' })).toBe(false)
+  expect(toolInputTouchesSecret({ file_path: 'src/app.ts' })).toBe(false)
+})
+
+test('toolInputTouchesSecret: safe on non-object / empty input', () => {
+  expect(toolInputTouchesSecret(null)).toBe(false)
+  expect(toolInputTouchesSecret(undefined)).toBe(false)
+  expect(toolInputTouchesSecret('a string')).toBe(false)
+  expect(toolInputTouchesSecret({})).toBe(false)
+})
diff --git a/src/lib/steps/harness/secret-paths.ts b/src/lib/steps/harness/secret-paths.ts
new file mode 100644
index 0000000..f973ff2
--- /dev/null
+++ b/src/lib/steps/harness/secret-paths.ts
@@ -0,0 +1,35 @@
+// Guards the integration agent away from the developer's secret files. `.env`
+// (and `.env.local`, `.env.production`, …) routinely hold secrets beyond the
+// Seam key — database URLs, Stripe keys, etc. — that must never be read into the
+// model context. `.env.example` is the value-less template the wizard writes, so
+// it stays readable.
+
+// Tool-input fields that carry a filesystem path across the built-in tools:
+// Read/Edit/Write use `file_path`, Grep/Glob/Ls use `path`, Grep also takes a
+// `glob`, and notebook tools use `notebook_path`.
+const PATH_BEARING_KEYS = [
+  'file_path',
+  'path',
+  'notebook_path',
+  'glob',
+] as const
+
+// True for `.env` and `.env.*` at any directory depth, except `.env.example`.
+export function isSecretFilePath(candidate: string): boolean {
+  const basename = candidate.split(/[/\\]/).pop() ?? candidate
+  if (basename === '.env.example') return false
+  return basename === '.env' || basename.startsWith('.env.')
+}
+
+// True when a tool's input targets a secret file via any of its path-bearing
+// fields. Used by the PreToolUse deny hook, which fires for every tool (so a
+// deny overrides the broad `Read` allow and covers Grep/Edit/Write too).
+export function toolInputTouchesSecret(toolInput: unknown): boolean {
+  if (typeof toolInput !== 'object' || toolInput == null) return false
+  const record = toolInput as Record<string, unknown>
+  for (const key of PATH_BEARING_KEYS) {
+    const value = record[key]
+    if (typeof value === 'string' && isSecretFilePath(value)) return true
+  }
+  return false
+}