diff --git a/src/index.ts b/src/index.ts
index 494d6944..9f94d378 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -14,6 +14,18 @@ import { program } from './cli.js';
 // In web mode, we should NOT exit on transient errors — log and continue
 const isWebMode = process.argv.includes('web');
 
+// COD-115: Codeman IS a tmux controller; it must never present as a tmux *client*.
+// If the web server is launched from inside a tmux pane it inherits TMUX/TMUX_PANE,
+// and tmux's nesting guard then kills every new attach-bridge PTY (exit 1 → respawn
+// loop, crash-looping any new tmux-backed session). Scrub at the root so every
+// downstream `{...process.env}` spread (attach, send-keys, create) is clean regardless
+// of launch context. `delete` (not `= undefined`, which node-pty serializes as the
+// literal string "undefined" and fails to clear).
+if (isWebMode) {
+  delete process.env.TMUX;
+  delete process.env.TMUX_PANE;
+}
+
 import { MAX_CONSECUTIVE_ERRORS, ERROR_RESET_MS } from './config/server-timing.js';
 
 // Track consecutive unhandled errors in web mode — restart after too many
diff --git a/src/session-cli-builder.ts b/src/session-cli-builder.ts
index ad6ed85f..72ee3f96 100644
--- a/src/session-cli-builder.ts
+++ b/src/session-cli-builder.ts
@@ -124,17 +124,32 @@ export function buildClaudeEnv(sessionId: string): Record<string, string | undef
  * Lighter than buildClaudeEnv — no PATH augmentation or Codeman vars needed
  * since the mux session already has those set.
  *
+ * @param truecolorEnabled - When true, set COLORTERM=truecolor (COD-75 opt-in);
+ *   otherwise leave COLORTERM unset. Mirrors buildEnvExports() so both paths agree.
  * @returns Environment variables object for pty.spawn
  */
-export function buildMuxAttachEnv(): Record<string, string | undefined> {
-  return {
+export function buildMuxAttachEnv(truecolorEnabled?: boolean): Record<string, string | undefined> {
+  const env: Record<string, string | undefined> = {
     ...process.env,
     LANG: 'en_US.UTF-8',
     LC_ALL: 'en_US.UTF-8',
     TERM: 'xterm-256color',
-    COLORTERM: undefined,
-    CLAUDECODE: undefined,
   };
+  // COD-115: keys to UNSET must be `delete`d, NOT set to `undefined`. On a
+  // `{...process.env}` spread the key stays present with value undefined, and node-pty
+  // serializes it as the literal string "TMUX=undefined" — a non-empty value that still
+  // trips tmux's nesting guard, killing the attach-bridge PTY (exit 1 → respawn loop).
+  // The server can be launched from inside tmux; attach clients must never inherit that
+  // parent tmux context. (Same fix the working create path uses in tmux-manager.ts.)
+  delete env.TMUX;
+  delete env.TMUX_PANE;
+  delete env.CLAUDECODE;
+  if (truecolorEnabled) {
+    env.COLORTERM = 'truecolor';
+  } else {
+    delete env.COLORTERM; // COD-75: unset for non-truecolor (was `: undefined`, same node-pty quirk)
+  }
+  return env;
 }
 
 /**
diff --git a/test/session-cli-builder.test.ts b/test/session-cli-builder.test.ts
new file mode 100644
index 00000000..f853d0ee
--- /dev/null
+++ b/test/session-cli-builder.test.ts
@@ -0,0 +1,66 @@
+/**
+ * @fileoverview Tests for CLI environment builders.
+ *
+ * Port: N/A (no server needed)
+ */
+
+import { describe, it, expect } from 'vitest';
+import { buildMuxAttachEnv } from '../src/session-cli-builder.js';
+
+describe('buildMuxAttachEnv', () => {
+  it('does not pass an inherited tmux context into tmux attach clients', () => {
+    const originalTmux = process.env.TMUX;
+    const originalTmuxPane = process.env.TMUX_PANE;
+    process.env.TMUX = '/tmp/tmux-1000/codeman,1169416,9';
+    process.env.TMUX_PANE = '%9';
+
+    try {
+      const env = buildMuxAttachEnv();
+
+      expect(env.TMUX).toBeUndefined();
+      expect(env.TMUX_PANE).toBeUndefined();
+    } finally {
+      if (originalTmux === undefined) {
+        delete process.env.TMUX;
+      } else {
+        process.env.TMUX = originalTmux;
+      }
+      if (originalTmuxPane === undefined) {
+        delete process.env.TMUX_PANE;
+      } else {
+        process.env.TMUX_PANE = originalTmuxPane;
+      }
+    }
+  });
+
+  // COD-115: `{...process.env, TMUX: undefined}` leaves the KEY present with value
+  // undefined; node-pty serializes that as the literal string "TMUX=undefined", which
+  // still trips tmux's nesting guard and kills the attach-bridge PTY (exit 1 → respawn
+  // loop). The keys must be genuinely ABSENT, which only `delete` achieves.
+  it('deletes tmux/claude context keys entirely (absent, not present-with-undefined) (COD-115)', () => {
+    const saved = {
+      TMUX: process.env.TMUX,
+      TMUX_PANE: process.env.TMUX_PANE,
+      CLAUDECODE: process.env.CLAUDECODE,
+    };
+    process.env.TMUX = '/tmp/tmux-1000/codeman,1169416,9';
+    process.env.TMUX_PANE = '%9';
+    process.env.CLAUDECODE = '1';
+
+    try {
+      const env = buildMuxAttachEnv();
+
+      expect('TMUX' in env).toBe(false);
+      expect('TMUX_PANE' in env).toBe(false);
+      expect('CLAUDECODE' in env).toBe(false);
+    } finally {
+      for (const [k, v] of Object.entries(saved)) {
+        if (v === undefined) {
+          delete process.env[k];
+        } else {
+          process.env[k] = v;
+        }
+      }
+    }
+  });
+});