fix(ai-config): force tab-width=8 in gptel org-mode prompt buffers

gptel's `gptel--with-buffer-copy-internal` copies the source buffer's `major-mode` symbol but doesn't run mode hooks. An inherited-org-mode prompt buffer keeps `tab-width` at this config's global default of 4 instead of the 8 that `org-mode-hook` would set. When gptel later parses the prompt buffer with `org-element`, Org's `tab-width=8` guard raises "Tab width in Org files must be 8, not 4."

I was hitting this on every second `gptel-magit-generate-message` from COMMIT_EDITMSG. `vc-config.el` sets `git-commit-major-mode 'org-mode'`, and the diffs contained list-shaped content that `org-element--list-struct` parsed.

The advice forces `tab-width=8` in the prompt buffer when its inherited mode is org-mode. It's a local workaround for an upstream gap. An upstream patch to run `(delay-mode-hooks (funcall major-mode))` in the buffer-copy is the real fix. I'll send it next.

2 files changed, 121 insertions, 0 deletions

diff --git a/tests/test-ai-config-gptel-prompt-tab-width.el b/tests/test-ai-config-gptel-prompt-tab-width.el
new file mode 100644
index 00000000..6f857cdc
--- /dev/null
+++ b/tests/test-ai-config-gptel-prompt-tab-width.el
@@ -0,0 +1,106 @@
+;;; test-ai-config-gptel-prompt-tab-width.el --- Tests for gptel prompt-buffer tab-width fix -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Regression tests for the `org-element--list-struct: Tab width in Org
+;; files must be 8, not 4' error that fires when `gptel-magit' is
+;; invoked from a `COMMIT_EDITMSG' buffer where `git-commit-major-mode'
+;; is `org-mode'.
+;;
+;; Root cause: gptel's `gptel--with-buffer-copy-internal'
+;; (gptel-request.el:945) sets the prompt buffer's `major-mode' to the
+;; source buffer's mode as a *symbol*, without running mode hooks.  So
+;; an inherited `org-mode' prompt buffer keeps the global default
+;; `tab-width' (4 in this config) — `org-mode-hook' never runs to set
+;; 8.  When gptel later parses the prompt buffer with `org-element',
+;; Org's `tab-width=8' guard raises.
+;;
+;; Fix: an `:around' advice on `gptel--with-buffer-copy-internal' in
+;; ai-config.el sets `tab-width=8' inside the prompt buffer when its
+;; inherited `major-mode' is `org-mode'.
+;;
+;; These tests verify the advice fires for org-mode source buffers and
+;; stays out of the way for other modes.  Each test pins
+;; `default-value' of `tab-width' to 4 — matching Craig's real config
+;; — so the advice's effect is observable in batch tests (where the
+;; default would otherwise be Emacs's built-in 8).
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; testutil-ai-config provides the gptel--with-buffer-copy-internal stub
+;; and the gptel stubs ai-config needs to load.
+(require 'testutil-ai-config)
+(require 'ai-config)
+
+(defmacro test-ai-config--with-default-tab-width (sentinel &rest body)
+  "Bind `default-value' of `tab-width' to SENTINEL during BODY.
+Restores the prior default on unwind.  New buffers created inside BODY
+inherit SENTINEL as their `tab-width'."
+  (declare (indent 1))
+  `(let ((saved (default-value 'tab-width)))
+     (unwind-protect
+         (progn (setq-default tab-width ,sentinel)
+                ,@body)
+       (setq-default tab-width saved))))
+
+;;; Normal Cases
+
+(ert-deftest test-ai-config-gptel-prompt-tab-width-org-mode-source-sets-8 ()
+  "Normal: source buffer in `org-mode' → prompt buffer has `tab-width=8'.
+Regression net for the `org-element--list-struct: Tab width must be 8'
+error when gptel-magit runs from COMMIT_EDITMSG with
+`git-commit-major-mode=org-mode'."
+  (test-ai-config--with-default-tab-width 4
+    (let ((recorded nil))
+      (with-temp-buffer
+        (setq major-mode 'org-mode)
+        (gptel--with-buffer-copy-internal
+         (current-buffer) nil nil
+         (lambda () (setq recorded tab-width))))
+      (should (= recorded 8)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-ai-config-gptel-prompt-tab-width-text-mode-source-not-overridden ()
+  "Boundary: source buffer in `text-mode' → advice does NOT set `tab-width=8'.
+Only `org-mode' source triggers the override; other modes keep the
+inherited default."
+  (test-ai-config--with-default-tab-width 4
+    (let ((recorded nil))
+      (with-temp-buffer
+        (setq major-mode 'text-mode)
+        (gptel--with-buffer-copy-internal
+         (current-buffer) nil nil
+         (lambda () (setq recorded tab-width))))
+      (should (= recorded 4)))))
+
+(ert-deftest test-ai-config-gptel-prompt-tab-width-fundamental-mode-source-not-overridden ()
+  "Boundary: source buffer in `fundamental-mode' → no override.
+Confirms the advice's `(eq major-mode 'org-mode)' gate is strict."
+  (test-ai-config--with-default-tab-width 4
+    (let ((recorded nil))
+      (with-temp-buffer
+        (setq major-mode 'fundamental-mode)
+        (gptel--with-buffer-copy-internal
+         (current-buffer) nil nil
+         (lambda () (setq recorded tab-width))))
+      (should (= recorded 4)))))
+
+(ert-deftest test-ai-config-gptel-prompt-tab-width-advice-does-not-leak ()
+  "Boundary: advice's `setq-local' stays buffer-local to the prompt buffer.
+After body-thunk completes and the prompt buffer is killed, the source
+buffer's `tab-width' is unchanged."
+  (with-temp-buffer
+    (setq major-mode 'org-mode)
+    (setq-local tab-width 4)
+    (gptel--with-buffer-copy-internal
+     (current-buffer) nil nil
+     (lambda () nil))
+    (should (= tab-width 4))))
+
+(provide 'test-ai-config-gptel-prompt-tab-width)
+;;; test-ai-config-gptel-prompt-tab-width.el ends here
diff --git a/tests/testutil-ai-config.el b/tests/testutil-ai-config.el
index c7486222..f56a38e1 100644
--- a/tests/testutil-ai-config.el
+++ b/tests/testutil-ai-config.el
@@ -74,6 +74,21 @@
 ;; so the magit integration only activates when magit is provided.
 ;; See test-ai-config-gptel-magit-lazy-loading.el for magit stub tests.
 
+;; Stub `gptel--with-buffer-copy-internal' so the advice attached to it
+;; in ai-config.el can be exercised without loading real gptel-request.
+;; Mirrors the salient bits of the real function (gptel-request.el:945):
+;; create a temp buffer, copy major-mode as a symbol (no hooks), call
+;; body-thunk inside.  See test-ai-config-gptel-prompt-tab-width.el.
+(unless (fboundp 'gptel--with-buffer-copy-internal)
+  (defun gptel--with-buffer-copy-internal (buf _start _end body-thunk)
+    "Stub: create temp buffer, copy major-mode from BUF, run BODY-THUNK."
+    (let ((temp (generate-new-buffer " *gptel-prompt-stub*" t)))
+      (unwind-protect
+          (with-current-buffer temp
+            (setq major-mode (buffer-local-value 'major-mode buf))
+            (funcall body-thunk))
+        (kill-buffer temp)))))
+
 ;; Stub ai-conversations
 (provide 'ai-conversations)