diff --git a/cmd/prdescribe.go b/cmd/prdescribe.go
index d489801..4313900 100644
--- a/cmd/prdescribe.go
+++ b/cmd/prdescribe.go
@@ -14,17 +14,23 @@ var prDescribeCmd = &cobra.Command{
 	Run:   runPRDescribe,
 }
 
+func init() {
+	prDescribeCmd.Flags().StringP("base", "b", "master", "Base branch to compare against")
+}
+
 func runPRDescribe(cmd *cobra.Command, args []string) {
-	diff, err := gitRun([]string{"diff", "main..HEAD", "--no-color"})
+	base, _ := cmd.Flags().GetString("base")
+
+	diff, err := gitRun([]string{"diff", fmt.Sprintf("%s..HEAD", base), "--no-color"})
 	if err != nil || diff == "" {
-		diff, err = gitRun([]string{"diff", "origin/main..HEAD", "--no-color"})
+		diff, err = gitRun([]string{"diff", fmt.Sprintf("origin/%s..HEAD", base), "--no-color"})
 		if err != nil {
 			color.Red("Failed to get branch diff: %v", err)
 			return
 		}
 	}
 	if diff == "" {
-		color.Yellow("No changes on this branch compared to main/origin/main.")
+		color.Yellow("No changes on this branch compared to %s/origin/%s.", base, base)
 		return
 	}
 	modelFlag, _ := cmd.Flags().GetString("model")
diff --git a/cmd/run_test.go b/cmd/run_test.go
index 770d170..859f40d 100644
--- a/cmd/run_test.go
+++ b/cmd/run_test.go
@@ -47,10 +47,11 @@ func withMockGit(fn func([]string) (string, error)) func() {
 	return func() { gitRun = orig }
 }
 
-// testCmd returns a minimal cobra command with the model flag registered.
+// testCmd returns a minimal cobra command with common flags registered.
 func testCmd() *cobra.Command {
 	c := &cobra.Command{}
 	c.Flags().String("model", "", "")
+	c.Flags().String("base", "master", "")
 	return c
 }
 
@@ -308,22 +309,62 @@ func TestRunPRDescribe(t *testing.T) {
 		}
 	})
 
-	t.Run("second diff error — skips AI", func(t *testing.T) {
-		mock := &mockStreamer{}
+	t.Run("uses custom base branch", func(t *testing.T) {
+		mock := &mockStreamer{response: "PR description"}
 		defer withMockClient(mock)()
-		callCount := 0
+		var capturedArgs []string
 		defer withMockGit(func(args []string) (string, error) {
-			callCount++
-			if callCount == 2 {
-				return "", errors.New("no remote")
+			capturedArgs = args
+			return "diff content", nil
+		})()
+
+		cmd := testCmd()
+		if err := cmd.Flags().Set("base", "develop"); err != nil {
+			t.Fatal(err)
+		}
+		runPRDescribe(cmd, nil)
+
+		if mock.calls != 1 {
+			t.Errorf("expected 1 AI call, got %d", mock.calls)
+		}
+		// Expect "diff", "develop..HEAD", "--no-color"
+		expectedArg := "develop..HEAD"
+		found := false
+		for _, arg := range capturedArgs {
+			if arg == expectedArg {
+				found = true
+				break
 			}
-			return "", nil
+		}
+		if !found {
+			t.Errorf("expected arg %q not found in %v", expectedArg, capturedArgs)
+		}
+	})
+
+	t.Run("defaults to master", func(t *testing.T) {
+		mock := &mockStreamer{response: "PR description"}
+		defer withMockClient(mock)()
+		var capturedArgs []string
+		defer withMockGit(func(args []string) (string, error) {
+			capturedArgs = args
+			return "diff content", nil
 		})()
 
 		runPRDescribe(testCmd(), nil)
 
-		if mock.calls != 0 {
-			t.Errorf("expected 0 AI calls, got %d", mock.calls)
+		if mock.calls != 1 {
+			t.Errorf("expected 1 AI call, got %d", mock.calls)
+		}
+		expectedArg := "master..HEAD"
+		found := false
+		for _, arg := range capturedArgs {
+			if arg == expectedArg {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("expected arg %q not found in %v", expectedArg, capturedArgs)
 		}
 	})
 }