diff --git a/.claude/hooks/dist/mcp-directory-guard.mjs b/.claude/hooks/dist/mcp-directory-guard.mjs
new file mode 100644
index 00000000..e50ce092
--- /dev/null
+++ b/.claude/hooks/dist/mcp-directory-guard.mjs
@@ -0,0 +1,86 @@
+// src/mcp-directory-guard.ts
+import { readFileSync } from "fs";
+
+// src/shared/opc-path.ts
+import { existsSync } from "fs";
+import { join } from "path";
+function getOpcDir() {
+  const envOpcDir = process.env.CLAUDE_OPC_DIR;
+  if (envOpcDir && existsSync(envOpcDir)) {
+    return envOpcDir;
+  }
+  const projectDir = process.env.CLAUDE_PROJECT_DIR || process.cwd();
+  const localOpc = join(projectDir, "opc");
+  if (existsSync(localOpc)) {
+    return localOpc;
+  }
+  const homeDir = process.env.HOME || process.env.USERPROFILE || "";
+  if (homeDir) {
+    const globalClaude = join(homeDir, ".claude");
+    const globalScripts = join(globalClaude, "scripts", "core");
+    if (existsSync(globalScripts)) {
+      return globalClaude;
+    }
+  }
+  return null;
+}
+
+// src/mcp-directory-guard.ts
+var SCRIPT_PATH_PATTERN = /\bscripts\/(mcp|core)\//;
+function buildCdPrefixPattern(opcDir) {
+  const escapedDir = opcDir ? opcDir.replace(/[.*+?^${}()|[\]\\]/g, "\\$&") : "";
+  const variants = [
+    "\\$CLAUDE_OPC_DIR",
+    "\\$\\{CLAUDE_OPC_DIR\\}"
+  ];
+  if (escapedDir) {
+    variants.push(escapedDir);
+  }
+  return new RegExp(`^\\s*cd\\s+(${variants.join("|")})\\s*&&`);
+}
+function main() {
+  let input;
+  try {
+    const stdinContent = readFileSync(0, "utf-8");
+    input = JSON.parse(stdinContent);
+  } catch {
+    console.log("{}");
+    return;
+  }
+  if (input.tool_name !== "Bash") {
+    console.log("{}");
+    return;
+  }
+  const command = input.tool_input?.command;
+  if (!command) {
+    console.log("{}");
+    return;
+  }
+  if (!SCRIPT_PATH_PATTERN.test(command)) {
+    console.log("{}");
+    return;
+  }
+  const opcDir = getOpcDir();
+  const cdPrefix = buildCdPrefixPattern(opcDir);
+  if (cdPrefix.test(command)) {
+    console.log("{}");
+    return;
+  }
+  const dirRef = opcDir || "$CLAUDE_OPC_DIR";
+  const corrected = `cd ${dirRef} && ${command.trimStart()}`;
+  const output = {
+    hookSpecificOutput: {
+      hookEventName: "PreToolUse",
+      permissionDecision: "deny",
+      permissionDecisionReason: `OPC directory guard: commands referencing scripts/(mcp|core)/ must run from the OPC directory so uv can find pyproject.toml.
+
+Blocked command:
+  ${command.trim()}
+
+Corrected command:
+  ${corrected}`
+    }
+  };
+  console.log(JSON.stringify(output));
+}
+main();
diff --git a/.claude/hooks/src/mcp-directory-guard.ts b/.claude/hooks/src/mcp-directory-guard.ts
new file mode 100644
index 00000000..6709b191
--- /dev/null
+++ b/.claude/hooks/src/mcp-directory-guard.ts
@@ -0,0 +1,104 @@
+/**
+ * PreToolUse:Bash Hook - OPC Script Directory Guard
+ *
+ * Prevents running scripts from `scripts/(mcp|core)/` without first
+ * changing to $CLAUDE_OPC_DIR. When Claude runs these scripts from the
+ * wrong directory, `uv run` misses `opc/pyproject.toml` and its
+ * dependencies, causing ModuleNotFoundError.
+ *
+ * Detection: any Bash command referencing `scripts/(mcp|core)/` paths
+ * Allowed: commands prefixed with `cd $CLAUDE_OPC_DIR &&` (or resolved path)
+ * Denied: returns corrected command in the reason message
+ *
+ * Fixes: #148
+ */
+
+import { readFileSync } from 'fs';
+import { getOpcDir } from './shared/opc-path.js';
+import type { PreToolUseInput, PreToolUseHookOutput } from './shared/types.js';
+
+/**
+ * Pattern matching scripts/(mcp|core)/ references in Bash commands.
+ * Captures the path for use in the corrected command suggestion.
+ */
+const SCRIPT_PATH_PATTERN = /\bscripts\/(mcp|core)\//;
+
+/**
+ * Pattern matching a proper cd prefix to OPC dir.
+ * Accepts:
+ *   cd $CLAUDE_OPC_DIR &&
+ *   cd ${CLAUDE_OPC_DIR} &&
+ *   cd /resolved/opc/path &&
+ */
+function buildCdPrefixPattern(opcDir: string | null): RegExp {
+  const escapedDir = opcDir ? opcDir.replace(/[.*+?^${}()|[\]\\]/g, '\\$&') : '';
+  // Match: cd <opc-dir-variant> && (with flexible whitespace)
+  const variants = [
+    '\\$CLAUDE_OPC_DIR',
+    '\\$\\{CLAUDE_OPC_DIR\\}',
+  ];
+  if (escapedDir) {
+    variants.push(escapedDir);
+  }
+  return new RegExp(`^\\s*cd\\s+(${variants.join('|')})\\s*&&`);
+}
+
+function main(): void {
+  let input: PreToolUseInput;
+  try {
+    const stdinContent = readFileSync(0, 'utf-8');
+    input = JSON.parse(stdinContent) as PreToolUseInput;
+  } catch {
+    // Can't read input - allow through
+    console.log('{}');
+    return;
+  }
+
+  // Only process Bash tool
+  if (input.tool_name !== 'Bash') {
+    console.log('{}');
+    return;
+  }
+
+  const command = input.tool_input?.command as string;
+  if (!command) {
+    console.log('{}');
+    return;
+  }
+
+  // Check if command references OPC script paths
+  if (!SCRIPT_PATH_PATTERN.test(command)) {
+    // No script path reference - allow through
+    console.log('{}');
+    return;
+  }
+
+  const opcDir = getOpcDir();
+  const cdPrefix = buildCdPrefixPattern(opcDir);
+
+  // Check if command already has the correct cd prefix
+  if (cdPrefix.test(command)) {
+    console.log('{}');
+    return;
+  }
+
+  // Build corrected command suggestion
+  const dirRef = opcDir || '$CLAUDE_OPC_DIR';
+  const corrected = `cd ${dirRef} && ${command.trimStart()}`;
+
+  const output: PreToolUseHookOutput = {
+    hookSpecificOutput: {
+      hookEventName: 'PreToolUse',
+      permissionDecision: 'deny',
+      permissionDecisionReason:
+        `OPC directory guard: commands referencing scripts/(mcp|core)/ must ` +
+        `run from the OPC directory so uv can find pyproject.toml.\n\n` +
+        `Blocked command:\n  ${command.trim()}\n\n` +
+        `Corrected command:\n  ${corrected}`,
+    },
+  };
+
+  console.log(JSON.stringify(output));
+}
+
+main();
diff --git a/.claude/settings.json b/.claude/settings.json
index c575d685..546caff2 100644
--- a/.claude/settings.json
+++ b/.claude/settings.json
@@ -77,6 +77,16 @@
             "timeout": 5
           }
         ]
+      },
+      {
+        "matcher": "Bash",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "node $HOME/.claude/hooks/dist/mcp-directory-guard.mjs",
+            "timeout": 5
+          }
+        ]
       }
     ],
     "PreCompact": [