diff --git a/apps/mcp/src/server/auth/index.test.ts b/apps/mcp/src/server/auth/index.test.ts
index 7adb52c4..62698e81 100644
--- a/apps/mcp/src/server/auth/index.test.ts
+++ b/apps/mcp/src/server/auth/index.test.ts
@@ -239,6 +239,48 @@ describe("MCP authentication", () => {
 		expect(warn).not.toHaveBeenCalled()
 	})
 
+	it("surfaces a 200 session body that fails the session contract as TransientAuthError", async () => {
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {})
+		const error = vi.spyOn(console, "error").mockImplementation(() => {})
+		vi.stubGlobal(
+			"fetch",
+			vi.fn().mockResolvedValue(
+				new Response(JSON.stringify({}), {
+					status: 200,
+					headers: { "content-type": "application/json" },
+				}),
+			),
+		)
+
+		await expect(
+			validateApiKey("sm_badbody_key_0123456789abcd", API_URL),
+		).rejects.toThrow(TransientAuthError)
+		expect(error).toHaveBeenCalledTimes(1)
+		expect(error.mock.calls[0]?.[0]).toBe(
+			"Auth backend unavailable during API key validation: Invalid session response",
+		)
+		expect(warn).not.toHaveBeenCalled()
+	})
+
+	it("surfaces a non-JSON 200 session body as TransientAuthError", async () => {
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {})
+		const error = vi.spyOn(console, "error").mockImplementation(() => {})
+		vi.stubGlobal(
+			"fetch",
+			vi
+				.fn()
+				.mockResolvedValue(
+					new Response("<html>upstream error</html>", { status: 200 }),
+				),
+		)
+
+		await expect(
+			validateApiKey("sm_htmlbody_key_0123456789abc", API_URL),
+		).rejects.toThrow(TransientAuthError)
+		expect(error).toHaveBeenCalledTimes(1)
+		expect(warn).not.toHaveBeenCalled()
+	})
+
 	it("surfaces a session-endpoint timeout as TransientAuthError", async () => {
 		const warn = vi.spyOn(console, "warn").mockImplementation(() => {})
 		const error = vi.spyOn(console, "error").mockImplementation(() => {})
diff --git a/apps/mcp/src/server/auth/index.ts b/apps/mcp/src/server/auth/index.ts
index b1e56bf9..a68d7c9f 100644
--- a/apps/mcp/src/server/auth/index.ts
+++ b/apps/mcp/src/server/auth/index.ts
@@ -44,9 +44,13 @@ export async function fetchSession(
 		)
 	}
 
-	const result = sessionInfoSchema.safeParse(await response.json())
+	const body = await response.json().catch((error: unknown) => {
+		if (error instanceof SyntaxError) return undefined
+		throw error
+	})
+	const result = sessionInfoSchema.safeParse(body)
 	if (!result.success) {
-		throw new Error("Invalid session response")
+		throw new TransientAuthError("Invalid session response")
 	}
 
 	return result.data
@@ -86,6 +90,7 @@ const TRANSIENT_ERROR_NAMES = new Set([
 const TRANSIENT_JOSE_CODES = new Set(["ERR_JWKS_TIMEOUT", "ERR_JOSE_GENERIC"])
 
 function transientAuthErrorFor(error: unknown): TransientAuthError | null {
+	if (error instanceof TransientAuthError) return error
 	const status = (error as { status?: unknown } | null)?.status
 	if (typeof status === "number" && status !== 401 && status !== 403) {
 		return new TransientAuthError(`Session endpoint returned ${status}`, status)