fix(error): surface the provider's reason on 400/422

A 400/422 collapsed to one generic localized line, discarding the body the provider actually returned (context-length exceeded, unpaired tool_calls, …). Append that reason for the request-shaped 4xx so the failure is diagnosable; other statuses keep their actionable fix text.

internal/control/errmsg.go

@@ -1,6 +1,7 @@
 package control
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 
@@ -17,10 +18,14 @@ func explainError(err error) error {
 	}
 	var apiErr *provider.APIError
 	if errors.As(err, &apiErr) {
-		if msg := i18n.M.ProviderStatusMessage(apiErr.Status); msg != "" {
-			return errors.New(msg)
+		msg := i18n.M.ProviderStatusMessage(apiErr.Status)
+		if msg == "" {
+			return err
 		}
-		return err
+		if reason := requestErrorReason(apiErr); reason != "" {
+			return fmt.Errorf("%s\n%s", msg, reason)
+		}
+		return errors.New(msg)
 	}
 	var authErr *provider.AuthError
 	if errors.As(err, &authErr) {
@@ -35,3 +40,38 @@ func explainError(err error) error {
 	}
 	return err
 }
+
+// requestErrorReason returns the provider's verbatim reason for request-shaped
+// 4xx (400/422) — the localized line names the category, the body names the
+// actual cause (context-length exceeded, unpaired tool_calls). Empty otherwise.
+func requestErrorReason(e *provider.APIError) string {
+	if e.Status != 400 && e.Status != 422 {
+		return ""
+	}
+	return providerBodyReason(e.Body)
+}
+
+// providerBodyReason pulls the human reason from an OpenAI/Anthropic-shaped error
+// body ({"error":{"message":…}}), falling back to the trimmed raw body.
+func providerBodyReason(body string) string {
+	if body == "" {
+		return ""
+	}
+	var parsed struct {
+		Error struct {
+			Message string `json:"message"`
+		} `json:"error"`
+	}
+	if json.Unmarshal([]byte(body), &parsed) == nil && parsed.Error.Message != "" {
+		return clampRunes(parsed.Error.Message, 800)
+	}
+	return clampRunes(body, 800)
+}
+
+func clampRunes(s string, max int) string {
+	r := []rune(s)
+	if len(r) <= max {
+		return s
+	}
+	return string(r[:max]) + "…"
+}

internal/control/errmsg_test.go

@@ -31,6 +31,21 @@ func TestExplainError(t *testing.T) {
 		}
 	}
 
+	jsonBody := explainError(&provider.APIError{Provider: "deepseek", Status: 400, Body: `{"error":{"message":"This model's maximum context length is 65536 tokens.","type":"invalid_request_error"}}`})
+	if !strings.Contains(jsonBody.Error(), i18n.M.ProviderErrBadRequest) || !strings.Contains(jsonBody.Error(), "maximum context length") {
+		t.Errorf("400 should append the provider reason from a JSON body, got %q", jsonBody.Error())
+	}
+
+	rawBody := explainError(&provider.APIError{Provider: "deepseek", Status: 422, Body: "some unparseable detail"})
+	if !strings.Contains(rawBody.Error(), "some unparseable detail") {
+		t.Errorf("422 should fall back to the raw body, got %q", rawBody.Error())
+	}
+
+	noLeak := explainError(&provider.APIError{Provider: "deepseek", Status: 429, Body: `{"error":{"message":"slow down"}}`})
+	if noLeak.Error() != i18n.M.ProviderErrRateLimited {
+		t.Errorf("429 body must not leak into the message, got %q", noLeak.Error())
+	}
+
 	plain := errors.New("some other failure")
 	if explainError(plain) != plain {
 		t.Error("unknown errors should pass through unchanged")