diff --git a/.claude-plugin/marketplace.json b/.claude-plugin/marketplace.json
index 36bd608..3bc3de6 100644
--- a/.claude-plugin/marketplace.json
+++ b/.claude-plugin/marketplace.json
@@ -9,7 +9,7 @@
       "source": {
         "source": "npm",
         "package": "@copilotkit/aimock",
-        "version": "^1.27.0"
+        "version": "^1.27.1"
       },
       "description": "Fixture authoring skill for @copilotkit/aimock — LLM, multimedia (image/TTS/transcription/video), MCP, A2A, AG-UI, vector, embeddings, structured output, sequential responses, streaming physics, record/replay, agent loop patterns, and debugging"
     }
diff --git a/.claude-plugin/plugin.json b/.claude-plugin/plugin.json
index c02577c..c80e672 100644
--- a/.claude-plugin/plugin.json
+++ b/.claude-plugin/plugin.json
@@ -1,6 +1,6 @@
 {
   "name": "aimock",
-  "version": "1.27.0",
+  "version": "1.27.1",
   "description": "Fixture authoring guidance for @copilotkit/aimock — LLM, multimedia, MCP, A2A, AG-UI, vector, and service mocking",
   "author": {
     "name": "CopilotKit"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 93a0892..c46c481 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+## [1.27.1] - 2026-05-22
+
 ### Fixed
 
 - **Router** — systemMessage array exact-match logic was unsatisfiable for 2+ needles; collapsed to substring matching. Added `elevenlabs-tts` and `translation` to endpoint compatibility filter.
@@ -13,6 +15,10 @@
 - **Helpers** — extended `resolveUsage` with Gemini-native token fields. Preserved error cause in `resolveResponse` factory rethrow. `buildEmbeddingResponse` accepts optional usage. `extractFormField` escapes regex metacharacters.
 - **Drift test infra** — retry logging with body consumption, broadened `redactUrl` to cover `api_key`/`apikey`/`token`/`access_token` patterns, URL threaded into error messages with redaction, `parseDataOnlySSE` [DONE] filter fix, `parseTypedSSE` multi-line data handling with null guards.
 - **Drift collector** — invoke vitest directly via npx to avoid pnpm stdout prefix breaking JSON parse; classify raw stack traces as infrastructure errors instead of crashing.
+- **AG-UI config loader** — removed `/.*/` catch-all regex fallback when `match.message` is absent; fixtures without a message pattern no longer shadow other fixtures.
+- **AG-UI input validation** — runtime check that `input.messages` is an array after JSON parse; returns 400 instead of confusing downstream 404.
+- **AG-UI SSE writer** — `writeAGUIEventStream` uses logger abstraction instead of `console.warn`; handles non-Error throws.
+- **Drift test helpers** — `parseDataOnlySSE` handles multi-line data blocks, aligned with `providers.ts` implementation.
 
 ## [1.27.0] - 2026-05-20
 
diff --git a/charts/aimock/Chart.yaml b/charts/aimock/Chart.yaml
index 0de02f9..5d39bd6 100644
--- a/charts/aimock/Chart.yaml
+++ b/charts/aimock/Chart.yaml
@@ -3,4 +3,4 @@ name: aimock
 description: Mock infrastructure for AI application testing (OpenAI, Anthropic, Gemini, MCP, A2A, vector)
 type: application
 version: 0.1.0
-appVersion: "1.27.0"
+appVersion: "1.27.1"
diff --git a/package.json b/package.json
index ecf94fb..9f9c624 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@copilotkit/aimock",
-  "version": "1.27.0",
+  "version": "1.27.1",
   "description": "Mock infrastructure for AI application testing — LLM APIs, image generation, text-to-speech, transcription, audio generation, video generation, MCP tools, A2A agents, AG-UI event streams, vector databases, search, rerank, and moderation. One package, one port, zero dependencies.",
   "license": "MIT",
   "keywords": [
diff --git a/src/__tests__/agui-mock.test.ts b/src/__tests__/agui-mock.test.ts
index a78cb23..75d9928 100644
--- a/src/__tests__/agui-mock.test.ts
+++ b/src/__tests__/agui-mock.test.ts
@@ -102,7 +102,9 @@ function postRaw(
 
 function aguiInput(userMessage: string, extra?: Partial<AGUIRunAgentInput>): AGUIRunAgentInput {
   return {
-    messages: [{ role: "user", content: userMessage }],
+    threadId: "test-thread",
+    runId: "test-run",
+    messages: [{ id: "msg-1", role: "user", content: userMessage }],
     ...extra,
   };
 }
diff --git a/src/__tests__/drift/helpers.ts b/src/__tests__/drift/helpers.ts
index 5b1cbb4..b8ff82b 100644
--- a/src/__tests__/drift/helpers.ts
+++ b/src/__tests__/drift/helpers.ts
@@ -87,9 +87,17 @@ export async function httpPostRaw(
 /** Parse data-only SSE blocks (OpenAI Chat Completions, Gemini). */
 export function parseDataOnlySSE(body: string): object[] {
   return body
+    .replace(/\r\n/g, "\n")
     .split("\n\n")
-    .filter((block) => block.startsWith("data: ") && !block.includes("[DONE]"))
-    .map((block) => JSON.parse(block.slice(6)));
+    .filter((block) => block.startsWith("data: ") && block.trim() !== "data: [DONE]")
+    .map((block) => {
+      // Rejoin continuation lines (data split across multiple lines)
+      const json = block
+        .split("\n")
+        .map((line) => (line.startsWith("data: ") ? line.slice(6) : line))
+        .join("");
+      return JSON.parse(json);
+    });
 }
 
 /** Parse typed SSE blocks with event: + data: (Anthropic, OpenAI Responses). */
diff --git a/src/agui-handler.ts b/src/agui-handler.ts
index acdc115..1c5606c 100644
--- a/src/agui-handler.ts
+++ b/src/agui-handler.ts
@@ -5,6 +5,7 @@
 import * as http from "node:http";
 import { randomUUID } from "node:crypto";
 
+import type { Logger } from "./logger.js";
 import type {
   AGUIRunAgentInput,
   AGUIFixtureMatch,
@@ -597,7 +598,7 @@ export function buildReasoningEncryptedValue(
 export async function writeAGUIEventStream(
   res: http.ServerResponse,
   events: AGUIEvent[],
-  opts?: { delayMs?: number; signal?: AbortSignal },
+  opts?: { delayMs?: number; signal?: AbortSignal; logger?: Logger },
 ): Promise<void> {
   const delayMs = opts?.delayMs ?? 0;
 
@@ -616,9 +617,25 @@ export async function writeAGUIEventStream(
       res.write(`data: ${JSON.stringify(stamped)}\n\n`);
     } catch (err) {
       if (err instanceof TypeError || err instanceof RangeError) {
-        console.warn("AG-UI SSE write failed (serialization):", (err as Error).message);
+        const msg = (err as Error).message;
+        if (opts?.logger) {
+          opts.logger.warn("AG-UI SSE write failed (serialization):", msg);
+        } else {
+          console.warn("AG-UI SSE write failed (serialization):", msg);
+        }
       } else if (err instanceof Error) {
-        console.warn("AG-UI SSE write failed:", err.message);
+        if (opts?.logger) {
+          opts.logger.warn("AG-UI SSE write failed:", err.message);
+        } else {
+          console.warn("AG-UI SSE write failed:", err.message);
+        }
+      } else {
+        const msg = String(err);
+        if (opts?.logger) {
+          opts.logger.warn("AG-UI SSE write failed:", msg);
+        } else {
+          console.warn("AG-UI SSE write failed:", msg);
+        }
       }
       break;
     }
diff --git a/src/agui-mock.ts b/src/agui-mock.ts
index 192b2f8..2da50e1 100644
--- a/src/agui-mock.ts
+++ b/src/agui-mock.ts
@@ -169,6 +169,17 @@ export class AGUIMock implements Mountable {
       return true;
     }
 
+    if (input.messages !== undefined && !Array.isArray(input.messages)) {
+      res.writeHead(400, { "Content-Type": "application/json" });
+      res.end(
+        JSON.stringify({
+          error: "Invalid input: 'messages' must be an array when provided",
+        }),
+      );
+      this.journalRequest(req, pathname, 400);
+      return true;
+    }
+
     const fixture = findFixture(input, this.fixtures);
 
     if (fixture) {
diff --git a/src/config-loader.ts b/src/config-loader.ts
index 2c26f71..a02e450 100644
--- a/src/config-loader.ts
+++ b/src/config-loader.ts
@@ -8,6 +8,7 @@ import type { ChaosConfig, RecordConfig } from "./types.js";
 import type { MCPToolDefinition, MCPPromptDefinition } from "./mcp-types.js";
 import type { A2AAgentDefinition, A2APart, A2AArtifact, A2AStreamEvent } from "./a2a-types.js";
 import type { AGUIEvent } from "./agui-types.js";
+import { buildTextResponse } from "./agui-handler.js";
 import { VectorMock } from "./vector-mock.js";
 import type { QueryResult } from "./vector-types.js";
 import { Logger } from "./logger.js";
@@ -226,7 +227,22 @@ export async function startFromConfig(
           );
         }
         if (f.text) {
-          agui.onMessage(f.match.message ?? /.*/, f.text, { delayMs: f.delayMs });
+          if (f.match.message !== undefined) {
+            agui.onMessage(f.match.message, f.text, { delayMs: f.delayMs });
+          } else {
+            // No message pattern — register via addFixture so it only matches
+            // on other criteria (toolCallId, toolName, stateKey) instead of
+            // becoming a catch-all that matches every request.
+            agui.addFixture({
+              match: {
+                toolCallId: f.match.toolCallId,
+                toolName: f.match.toolName,
+                stateKey: f.match.stateKey,
+              },
+              events: buildTextResponse(f.text),
+              delayMs: f.delayMs,
+            });
+          }
         } else if (f.events) {
           agui.addFixture({
             match: {