feat(push): support generated messages on Windows

README.md

@@ -539,14 +539,14 @@ Example:
 BRAID_PUSH_COMMIT_MESSAGE_COMMAND='codex exec -C {REPO_DIR} --add-dir {CONTEXT_DIR} --model gpt-5.5 -c '\''model_reasoning_effort="low"'\'' -o {MESSAGE_FILE} < {PROMPT_FILE}'
 ```
 
-The command runs as `/bin/sh -c` from the downstream repository root with the
-current process environment. The prompt includes mirror metadata, downstream
+The command runs as `<git-shell> -c` from the downstream repository root with
+the current process environment. Braid obtains the POSIX shell path from
+`git var GIT_SHELL_PATH` and verifies that it starts before pushing; this also
+supports Git for Windows. The prompt includes mirror metadata, downstream
 commit provenance when it can be collected, and the staged upstream diff. Diffs
 up to 5 KiB are included inline; larger diffs are written under
 `{CONTEXT_DIR}` and referenced from the prompt. The configured command is
-trusted local shell code and is not sandboxed by Braid. On Windows, configured
-generation is not supported; leave the environment variable unset or empty to
-use the normal editor flow.
+trusted local shell code and is not sandboxed by Braid.
 
 When generation succeeds, Git's editor opens with the generated message followed
 by commented provenance guidance when available. The editor-reviewed content is

docs/migration-from-ruby-braid.md

@@ -150,8 +150,8 @@ tool keeps that model but tightens the user-facing behavior:
   without opening the editor or running the generated-message command.
 - `BRAID_PUSH_COMMIT_MESSAGE_COMMAND` can optionally run a trusted local POSIX
   shell command to generate a draft upstream commit message. The editor still
-  opens for review. This generator is disabled by default and is not supported
-  on Windows.
+  opens for review. This generator is disabled by default. Braid obtains the
+  shell from `git var GIT_SHELL_PATH`, including on Git for Windows.
 
 Migration impact:
 

integration/sync_test.go

@@ -93,6 +93,37 @@ func TestExecutablePushProvenanceTemplateTouchesGitDefaultTemplate(t *testing.T)
 	assertNotContains(t, template, "BRAID_COMMIT_TEMPLATE")
 }
 
+func TestExecutablePushGeneratesMessageWithGitPOSIXShell(t *testing.T) {
+	root := t.TempDir()
+	env := newProcessEnv(t, root)
+	braid := braidBinary(t)
+
+	upstream := filepath.Join(root, "upstream")
+	initRepo(t, env, upstream)
+	writeFile(t, upstream, "README.md", "base\n")
+	commitAll(t, env, upstream, "seed upstream")
+	gitOK(t, env, upstream, "config", "--local", "receive.denyCurrentBranch", "updateInstead")
+
+	downstream := filepath.Join(root, "downstream")
+	initRepo(t, env, downstream)
+	writeFile(t, downstream, "README.md", "downstream\n")
+	commitAll(t, env, downstream, "seed downstream")
+	add := runBraid(t, env, downstream, braid, "--quiet", "add", upstream, "vendor/basic")
+	assertResult(t, add, 0, "", "")
+
+	writeFile(t, downstream, "vendor/basic/README.md", "local\n")
+	commitAll(t, env, downstream, "local mirror change")
+	capture, editor := capturingEditorCommand(t, root, "Reviewed generated message")
+	pushEnv := env.
+		with("BRAID_PUSH_COMMIT_MESSAGE_COMMAND", "printf 'Generated by Git POSIX shell\\n' > {MESSAGE_FILE}").
+		with("GIT_EDITOR", editor)
+
+	push := runBraid(t, pushEnv, downstream, braid, "push", "vendor/basic")
+	assertExit(t, push, 0)
+	assertContains(t, readFile(t, root, filepath.Base(capture)), "Generated by Git POSIX shell")
+	assertLatestCommit(t, env, upstream, defaultName+" <"+defaultEmail+">", "Reviewed generated message")
+}
+
 func TestExecutableSyncPullOnlyUpdatesWithoutEditor(t *testing.T) {
 	root := t.TempDir()
 	env := newProcessEnv(t, root)

internal/command/preflight.go

@@ -108,6 +108,7 @@ type PushGit interface {
 	UpdateGit
 	ConfigGet(context.Context, ...string) (string, bool, error)
 	CoreCommentChar(context.Context) (string, bool, error)
+	ShellPath(context.Context) (string, error)
 	FirstParentCommits(context.Context, string) ([]string, error)
 	LogCommitsTouchingPath(context.Context, string, string) ([]gitexec.Commit, error)
 	ShowFile(context.Context, string, string) ([]byte, bool, error)

internal/command/push.go

@@ -129,7 +129,11 @@ func (h PushHandler) Run(inv cli.Invocation, stdout, stderr io.Writer) error {
 	if err != nil {
 		return err
 	}
-	result, err := h.push(ctx, repo, git, selection.Source.WithMirror(selection.Mirrors[0]), inv.Push.Branch, inv.Push.Keep, inv.Push.Message, inv.Global, stdout, stderr)
+	messageGeneration := pushMessageGeneration{}
+	if inv.Push.Message == "" {
+		messageGeneration = configuredPushMessageGeneration()
+	}
+	result, err := h.push(ctx, repo, git, selection.Source.WithMirror(selection.Mirrors[0]), inv.Push.Branch, inv.Push.Keep, inv.Push.Message, messageGeneration, inv.Global, stdout, stderr)
 	if err != nil {
 		return err
 	}
@@ -152,7 +156,7 @@ func (h PushHandler) pushGit(repo RepoContext, inv cli.Invocation, trace io.Writ
 	return gitexec.New(repo.GitWorkTreeRoot, inv.Global.Verbose, trace)
 }
 
-func (h PushHandler) push(ctx context.Context, repo RepoContext, git PushGit, m source.SourceMirror, branch string, keep bool, commitMessage string, global cli.GlobalOptions, stdout, stderr io.Writer) (result pushResult, err error) {
+func (h PushHandler) push(ctx context.Context, repo RepoContext, git PushGit, m source.SourceMirror, branch string, keep bool, commitMessage string, messageGeneration pushMessageGeneration, global cli.GlobalOptions, stdout, stderr io.Writer) (result pushResult, err error) {
 	if branch == "" {
 		branch = m.Branch()
 	}
@@ -255,13 +259,15 @@ func (h PushHandler) push(ctx context.Context, repo RepoContext, git PushGit, m
 	var provenance pushProvenance
 	var provenanceOK bool
 	var provenanceErr error
-	messageGeneration := pushMessageGeneration{}
 	if commitMessage == "" {
 		provenance, provenanceOK, provenanceErr = buildPushProvenance(ctx, git, m)
 		if provenanceErr != nil {
 			warnPushProvenance(stderr, provenanceErr)
 		}
-		messageGeneration = configuredPushMessageGeneration()
+		messageGeneration, err = resolvePushMessageGeneration(ctx, git, messageGeneration)
+		if err != nil {
+			return pushResult{}, err
+		}
 	}
 
 	pushCompleted := false

internal/command/push_message.go

@@ -8,7 +8,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"unicode/utf8"
 
@@ -17,20 +16,20 @@ import (
 )
 
 const (
-	pushMessageCommandEnv            = "BRAID_PUSH_COMMIT_MESSAGE_COMMAND"
-	pushMessageInlineDiffLimit       = 5 * 1024
-	pushMessageGeneratorOutputLimit  = 4 * 1024
-	pushMessageTruncationMarker      = "[truncated after 4096 bytes]"
-	pushMessagePromptFileName        = "prompt.txt"
-	pushMessageOutputFileName        = "message.txt"
-	pushMessageSeedFileName          = "commit-message-seed.txt"
-	pushMessageLargeDiffFileName     = "upstream.diff"
-	pushMessageUnsupportedWindowsMsg = "AI push commit-message generation requires POSIX /bin/sh support and is not supported on Windows; unset BRAID_PUSH_COMMIT_MESSAGE_COMMAND to push without generation"
+	pushMessageCommandEnv           = "BRAID_PUSH_COMMIT_MESSAGE_COMMAND"
+	pushMessageInlineDiffLimit      = 5 * 1024
+	pushMessageGeneratorOutputLimit = 4 * 1024
+	pushMessageTruncationMarker     = "[truncated after 4096 bytes]"
+	pushMessagePromptFileName       = "prompt.txt"
+	pushMessageOutputFileName       = "message.txt"
+	pushMessageSeedFileName         = "commit-message-seed.txt"
+	pushMessageLargeDiffFileName    = "upstream.diff"
 )
 
 type pushMessageGeneration struct {
 	Enabled         bool
 	CommandTemplate string
+	ShellPath       string
 }
 
 type pushMessageCommandValues struct {
@@ -79,9 +78,6 @@ func configuredPushMessageGeneration() pushMessageGeneration {
 }
 
 func preparePushMessageSeed(ctx context.Context, repo RepoContext, source PushGit, tempGit gitexec.Git, m source.SourceMirror, branch, baseRevision, newTree, contextDir string, generation pushMessageGeneration, verbose bool, trace io.Writer, provenance pushProvenance, provenanceOK bool, provenanceErr error) (string, error) {
-	if err := validatePushMessageGeneratorPlatform(runtime.GOOS); err != nil {
-		return "", err
-	}
 	commentChar, err := pushMessageSeedCommentChar(ctx, source)
 	if err != nil {
 		return "", err
@@ -124,7 +120,7 @@ func preparePushMessageSeed(ctx context.Context, repo RepoContext, source PushGi
 	if _, err := fmt.Fprintf(progress, "Braid: generating push commit message for %s using external tool\n", m.LocalPath); err != nil {
 		return "", err
 	}
-	generated, failure, err := runPushMessageGenerator(ctx, generation.CommandTemplate, pushMessageCommandValues{
+	generated, failure, err := runPushMessageGenerator(ctx, generation.ShellPath, generation.CommandTemplate, pushMessageCommandValues{
 		RepoDir:     repo.GitWorkTreeRoot,
 		ContextDir:  contextDir,
 		PromptFile:  promptPath,
@@ -146,13 +142,6 @@ func preparePushMessageSeed(ctx context.Context, repo RepoContext, source PushGi
 	return seedPath, nil
 }
 
-func validatePushMessageGeneratorPlatform(goos string) error {
-	if goos == "windows" {
-		return errors.New(pushMessageUnsupportedWindowsMsg)
-	}
-	return nil
-}
-
 func pushMessageSeedCommentChar(ctx context.Context, git PushGit) (string, error) {
 	value, ok, err := git.CoreCommentChar(ctx)
 	if err != nil {
@@ -259,17 +248,42 @@ func formatPushMessagePromptProvenance(provenance pushProvenance, ok bool, err e
 	return b.String()
 }
 
-func runPushMessageGenerator(ctx context.Context, commandTemplate string, values pushMessageCommandValues, verbose bool, trace io.Writer) (string, *pushMessageGeneratorFailure, error) {
+func resolvePushMessageGeneration(ctx context.Context, git PushGit, generation pushMessageGeneration) (pushMessageGeneration, error) {
+	if !generation.Enabled || generation.ShellPath != "" {
+		return generation, nil
+	}
+	shellPath, err := git.ShellPath(ctx)
+	if err != nil {
+		return pushMessageGeneration{}, fmt.Errorf("resolve POSIX shell with git var GIT_SHELL_PATH: %w", err)
+	}
+	if shellPath == "" {
+		return pushMessageGeneration{}, errors.New("resolve POSIX shell with git var GIT_SHELL_PATH: Git returned an empty path")
+	}
+	if err := probePushMessageShell(ctx, shellPath); err != nil {
+		return pushMessageGeneration{}, err
+	}
+	generation.ShellPath = shellPath
+	return generation, nil
+}
+
+func probePushMessageShell(ctx context.Context, shellPath string) error {
+	if err := exec.CommandContext(ctx, shellPath, "-c", ":").Run(); err != nil {
+		return fmt.Errorf("start Git POSIX shell %q: %w", shellPath, err)
+	}
+	return nil
+}
+
+func runPushMessageGenerator(ctx context.Context, shellPath, commandTemplate string, values pushMessageCommandValues, verbose bool, trace io.Writer) (string, *pushMessageGeneratorFailure, error) {
 	command := expandPushMessageCommand(commandTemplate, values)
 	if verbose {
 		if trace == nil {
 			trace = io.Discard
 		}
-		if _, err := fmt.Fprintf(trace, "Braid: Executing %s in %s\n", gitexec.FormatArgv([]string{"/bin/sh", "-c", command}), values.RepoDir); err != nil {
+		if _, err := fmt.Fprintf(trace, "Braid: Executing %s in %s\n", gitexec.FormatArgv([]string{shellPath, "-c", command}), values.RepoDir); err != nil {
 			return "", nil, err
 		}
 	}
-	cmd := exec.CommandContext(ctx, "/bin/sh", "-c", command)
+	cmd := exec.CommandContext(ctx, shellPath, "-c", command)
 	cmd.Dir = values.RepoDir
 	var stdout, stderr limitedOutput
 	stdout.limit = pushMessageGeneratorOutputLimit

internal/command/push_test.go

@@ -6,11 +6,11 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 
 	"braid/internal/config"
+	"braid/internal/gitexec"
 	"braid/internal/testutil"
 )
 
@@ -365,10 +365,6 @@ func TestPushCommandDoesNotPushWhenEditorFails(t *testing.T) {
 }
 
 func TestPushCommandGeneratedMessagePromptAndReview(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("push message generation requires POSIX /bin/sh")
-	}
-
 	upstream := testutil.InitRepo(t)
 	testutil.WriteFile(t, upstream, "README.md", "base\n")
 	testutil.CommitAll(t, upstream, "base")
@@ -405,7 +401,7 @@ func TestPushCommandGeneratedMessagePromptAndReview(t *testing.T) {
 	if err != nil {
 		t.Fatalf("resolve repo path: %v", err)
 	}
-	if got := strings.TrimSpace(readTestFile(t, repoCapture)); got != expectedRepo {
+	if got := filepath.Clean(filepath.FromSlash(strings.TrimSpace(readTestFile(t, repoCapture)))); got != expectedRepo {
 		t.Fatalf("generator repo arg = %q, want %q", got, expectedRepo)
 	}
 	prompt := readTestFile(t, promptCapture)
@@ -439,10 +435,6 @@ func TestPushCommandGeneratedMessagePromptAndReview(t *testing.T) {
 }
 
 func TestPushCommandGeneratorFailuresOpenEditorWithDiagnostics(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("push message generation requires POSIX /bin/sh")
-	}
-
 	tests := []struct {
 		name       string
 		body       string
@@ -510,10 +502,6 @@ func TestPushCommandGeneratorFailuresOpenEditorWithDiagnostics(t *testing.T) {
 }
 
 func TestPushCommandGeneratorReadsLargeDiffFromContextDir(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("push message generation requires POSIX /bin/sh")
-	}
-
 	upstream := testutil.InitRepo(t)
 	testutil.WriteFile(t, upstream, "README.md", "base\n")
 	testutil.CommitAll(t, upstream, "base")
@@ -528,7 +516,7 @@ func TestPushCommandGeneratorReadsLargeDiffFromContextDir(t *testing.T) {
 	captureDir := t.TempDir()
 	promptCapture := filepath.Join(captureDir, "prompt.txt")
 	diffCapture := filepath.Join(captureDir, "upstream.diff")
-	generator := writeGenerator(t, "#!/bin/sh\nprompt=$1\nmessage=$2\ncontext=$3\ndiff_file=$(sed -n 's/^Full diff file: //p' \"$prompt\")\n[ -n \"$diff_file\" ] || exit 12\n[ -f \"$diff_file\" ] || exit 13\ncase \"$diff_file\" in \"$context\"/*) ;; *) exit 14 ;; esac\ncp \"$prompt\" \"$BRAID_GENERATOR_PROMPT\" || exit 1\ncp \"$diff_file\" \"$BRAID_GENERATOR_DIFF\" || exit 1\nprintf 'Generated large diff\\n' > \"$message\"\n")
+	generator := writeGenerator(t, "#!/bin/sh\nprompt=$1\nmessage=$2\ncontext=$3\ndiff_file=$(sed -n 's/^Full diff file: //p' \"$prompt\")\n[ -n \"$diff_file\" ] || exit 12\n[ -f \"$diff_file\" ] || exit 13\nif command -v cygpath >/dev/null 2>&1; then\n  diff_file=$(cygpath -u \"$diff_file\")\n  context=$(cygpath -u \"$context\")\nfi\ncase \"$diff_file\" in \"$context\"/*) ;; *) exit 14 ;; esac\ncp \"$prompt\" \"$BRAID_GENERATOR_PROMPT\" || exit 1\ncp \"$diff_file\" \"$BRAID_GENERATOR_DIFF\" || exit 1\nprintf 'Generated large diff\\n' > \"$message\"\n")
 	t.Setenv("BRAID_GENERATOR_PROMPT", promptCapture)
 	t.Setenv("BRAID_GENERATOR_DIFF", diffCapture)
 	t.Setenv(pushMessageCommandEnv, shellQuote(generator)+" {PROMPT_FILE} {MESSAGE_FILE} {CONTEXT_DIR}")
@@ -549,10 +537,6 @@ func TestPushCommandGeneratorReadsLargeDiffFromContextDir(t *testing.T) {
 }
 
 func TestPushCommandGenerationContinuesWhenProvenanceFails(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("push message generation requires POSIX /bin/sh")
-	}
-
 	upstream := testutil.InitRepo(t)
 	testutil.WriteFile(t, upstream, "README.md", "base\n")
 	testutil.CommitAll(t, upstream, "base")
@@ -1031,10 +1015,6 @@ func TestPushMessageCommandSubstitutionQuotesDocumentedPlaceholders(t *testing.T
 }
 
 func TestPushMessageGeneratorVerboseTrace(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("push message generation requires POSIX /bin/sh")
-	}
-
 	repoDir := t.TempDir()
 	contextDir := t.TempDir()
 	promptPath := filepath.Join(contextDir, pushMessagePromptFileName)
@@ -1050,17 +1030,18 @@ func TestPushMessageGeneratorVerboseTrace(t *testing.T) {
 		PromptFile:  promptPath,
 		MessageFile: messagePath,
 	}
+	shellPath := strings.TrimSpace(testutil.Git(t, repoDir, "var", "GIT_SHELL_PATH").Stdout)
 
 	var quietTrace bytes.Buffer
-	if _, _, err := runPushMessageGenerator(context.Background(), template, values, false, &quietTrace); err != nil {
+	if _, _, err := runPushMessageGenerator(context.Background(), shellPath, template, values, false, &quietTrace); err != nil {
 		t.Fatalf("runPushMessageGenerator quiet returned error: %v", err)
 	}
 	if quietTrace.String() != "" {
 		t.Fatalf("quiet trace = %q, want empty", quietTrace.String())
 	}
 
 	var trace bytes.Buffer
-	message, failure, err := runPushMessageGenerator(context.Background(), template, values, true, &trace)
+	message, failure, err := runPushMessageGenerator(context.Background(), shellPath, template, values, true, &trace)
 	if err != nil {
 		t.Fatalf("runPushMessageGenerator verbose returned error: %v", err)
 	}
@@ -1070,10 +1051,11 @@ func TestPushMessageGeneratorVerboseTrace(t *testing.T) {
 	if message != "generated" {
 		t.Fatalf("generated message = %q, want generated", message)
 	}
-	assertContains(t, trace.String(), "Braid: Executing [\"/bin/sh\", \"-c\", ")
-	assertContains(t, trace.String(), shellQuote(generator))
-	assertContains(t, trace.String(), shellQuote(promptPath))
-	assertContains(t, trace.String(), shellQuote(messagePath))
+	assertContains(t, trace.String(), "Braid: Executing")
+	assertContains(t, trace.String(), shellPath)
+	assertContains(t, trace.String(), filepath.Base(generator))
+	assertContains(t, trace.String(), filepath.Base(promptPath))
+	assertContains(t, trace.String(), filepath.Base(messagePath))
 	assertContains(t, trace.String(), " in "+repoDir+"\n")
 }
 
@@ -1111,13 +1093,25 @@ func TestPushMessageDiffContextCutoff(t *testing.T) {
 	}
 }
 
-func TestPushMessageGeneratorPlatformSupport(t *testing.T) {
-	if err := validatePushMessageGeneratorPlatform("linux"); err != nil {
-		t.Fatalf("linux platform validation returned error: %v", err)
+func TestResolvePushMessageGenerationUsesGitShell(t *testing.T) {
+	git := gitexec.New(t.TempDir(), false, nil)
+	want, err := git.ShellPath(context.Background())
+	if err != nil {
+		t.Fatalf("ShellPath returned error: %v", err)
+	}
+	generation, err := resolvePushMessageGeneration(context.Background(), git, pushMessageGeneration{Enabled: true, CommandTemplate: "generate"})
+	if err != nil {
+		t.Fatalf("resolvePushMessageGeneration returned error: %v", err)
+	}
+	if generation.ShellPath != want {
+		t.Fatalf("resolved shell = %q, want %q", generation.ShellPath, want)
 	}
-	err := validatePushMessageGeneratorPlatform("windows")
-	if err == nil || !strings.Contains(err.Error(), "/bin/sh") || !strings.Contains(err.Error(), pushMessageCommandEnv) {
-		t.Fatalf("windows platform validation error = %v, want clear unsupported message", err)
+}
+
+func TestProbePushMessageShellRejectsMissingExecutable(t *testing.T) {
+	err := probePushMessageShell(context.Background(), filepath.Join(t.TempDir(), "missing-sh"))
+	if err == nil || !strings.Contains(err.Error(), "start Git POSIX shell") {
+		t.Fatalf("probePushMessageShell error = %v, want launch failure", err)
 	}
 }