fix: render failed-response retry refusals as readable text

Decode only Retry's HTTP 400 error field for the existing banner. Preserve raw model/key refusal envelopes and other submission-error handling.

Verified on the accepted-main integration: 2424 full-suite tests, typecheck, full ESLint, production build and isolated synthetic production-browser Retry/Submit smoke. No real provider calls.

components/session-core.ts

@@ -715,10 +715,17 @@ export class SessionModel {
 				throw new Error("A turn is already running — try again in a moment.");
 			}
 			if (!res.ok) {
-				// Refusal envelopes (503 BYOK/key gates) and 400s: the RAW body
+				// Keep model/key refusal envelopes verbatim: the page's refusal
-				// text rides the error message — the banner shows it, and the
+				// flow parses them. Retry's 400s are ordinary user-facing
-				// page's refusal flow JSON-parses the envelope out of it.
+				// guidance, so show their error text without JSON syntax.
-				throw new Error((await res.text()) || `The send failed (${res.status}).`);
+				let message = (await res.text()) || `The send failed (${res.status}).`;
+				if (input.type === "retry" && res.status === 400) {
+					try {
+						const refusal = JSON.parse(message) as { error?: unknown } | null;
+						if (typeof refusal?.error === "string" && refusal.error.length > 0) message = refusal.error;
+					} catch { /* Non-envelope failures keep their original text. */ }
+				}
+				throw new Error(message);
 			}
 			// CONNECTION DIET B (owner, 2026-10-10 — parallel-session freeze
 			// postmortem): the POST acknowledges and closes; there is no stream

tests/retry-failed-turn.test.ts

@@ -487,11 +487,27 @@ describe("the Retry button's client path", () => {
 		});
 		await m.send({ type: "retry" });
 		assert.equal(m.getState().status, "error");
-		assert.equal(JSON.parse(m.getState().error!.message).error, refused, "the existing refusal-envelope path carries both messages");
+		assert.equal(m.getState().error!.message, refused, "the banner keeps both messages as readable text, without JSON syntax");
 		assert.deepEqual(m.getState().nativeMessages, compacted);
 		assert.equal(endsInFailedTurn(m.getState().nativeMessages), false);
 	});
 
+	for (const { status, body, expected } of [
+		{ status: 400, body: '{"error":"retry input requires a failed turn to retry"}', expected: "retry input requires a failed turn to retry" },
+		{ status: 400, body: "Plain refusal", expected: "Plain refusal" },
+		{ status: 400, body: '{"reason":"not an error envelope"}', expected: '{"reason":"not an error envelope"}' },
+		{ status: 503, body: '{"error":"model gate","code":"models_not_configured"}', expected: '{"error":"model gate","code":"models_not_configured"}' },
+	]) {
+		it(`preserves the appropriate Retry refusal format for ${status}: ${body}`, async () => {
+			const m = new SessionModel({
+				chatId: "chat_x", initialMessages: [newUserMessage("hello"), failed("failed-response")],
+				fetch: async () => new Response(body, { status }),
+			});
+			await m.send({ type: "retry" });
+			assert.equal(m.getState().error?.message, expected);
+		});
+	}
+
 	it("after a dropped request, Retry posts straight from the error status", async () => {
 		const failedTurn = [newUserMessage("hello"), failed("a1")];
 		const posted: string[] = [];