diff --git a/core/tools/systemMessageTools/detectToolCallStart.ts b/core/tools/systemMessageTools/detectToolCallStart.ts
index e143411dda6..d96ed1d1fac 100644
--- a/core/tools/systemMessageTools/detectToolCallStart.ts
+++ b/core/tools/systemMessageTools/detectToolCallStart.ts
@@ -1,15 +1,25 @@
 import { SystemMessageToolsFramework } from "./types";
 
+interface DetectToolCallStartOptions {
+  allowAlternateStarts?: boolean;
+}
+
 export function detectToolCallStart(
   buffer: string,
   toolCallFramework: SystemMessageToolsFramework,
+  options: DetectToolCallStartOptions = {},
 ) {
+  const allowAlternateStarts = options.allowAlternateStarts ?? true;
   const starts = toolCallFramework.acceptedToolCallStarts;
   let modifiedBuffer = buffer;
   let isInToolCall = false;
   let isInPartialStart = false;
   const lowerCaseBuffer = buffer.toLowerCase();
   for (let i = 0; i < starts.length; i++) {
+    if (i !== 0 && !allowAlternateStarts) {
+      continue;
+    }
+
     const [start, replacement] = starts[i];
     if (lowerCaseBuffer.startsWith(start)) {
       // for non-standard cases like no ```tool codeblock, etc, replace before adding to buffer, case insensitive
diff --git a/core/tools/systemMessageTools/interceptSystemToolCalls.ts b/core/tools/systemMessageTools/interceptSystemToolCalls.ts
index a89fbdcfbb5..bf8fe4266d3 100644
--- a/core/tools/systemMessageTools/interceptSystemToolCalls.ts
+++ b/core/tools/systemMessageTools/interceptSystemToolCalls.ts
@@ -28,6 +28,7 @@ export async function* interceptSystemToolCalls(
 ): AsyncGenerator<ChatMessage[], PromptLog | undefined> {
   let buffer = "";
   let parseState: ToolCallParseState | undefined;
+  let sawAssistantNonWhitespaceText = false;
 
   while (true) {
     const result = await messageGenerator.next();
@@ -71,7 +72,10 @@ export async function* interceptSystemToolCalls(
           buffer += chunk;
           if (!parseState) {
             const { isInPartialStart, isInToolCall, modifiedBuffer } =
-              detectToolCallStart(buffer, systemToolFramework);
+              detectToolCallStart(buffer, systemToolFramework, {
+                // Only allow loose "TOOL_NAME:" starts at the beginning of assistant output.
+                allowAlternateStarts: !sawAssistantNonWhitespaceText,
+              });
 
             if (isInPartialStart) {
               continue;
@@ -109,6 +113,9 @@ export async function* interceptSystemToolCalls(
                 content: [{ type: "text", text: buffer }],
               },
             ];
+            if (/\S/.test(buffer)) {
+              sawAssistantNonWhitespaceText = true;
+            }
           }
           buffer = "";
         }
diff --git a/core/tools/systemMessageTools/toolCodeblocks/detectToolCallStart.vitest.ts b/core/tools/systemMessageTools/toolCodeblocks/detectToolCallStart.vitest.ts
index 297c58ec456..0adf6d0c321 100644
--- a/core/tools/systemMessageTools/toolCodeblocks/detectToolCallStart.vitest.ts
+++ b/core/tools/systemMessageTools/toolCodeblocks/detectToolCallStart.vitest.ts
@@ -75,4 +75,15 @@ describe("detectToolCallStart", () => {
     expect(result.isInPartialStart).toBe(false);
     expect(result.modifiedBuffer).toBe(buffer);
   });
+
+  it("skips non-standard starts when alternate starts are disabled", () => {
+    const buffer = "TOOL_NAME: example_tool";
+    const result = detectToolCallStart(buffer, framework, {
+      allowAlternateStarts: false,
+    });
+
+    expect(result.isInToolCall).toBe(false);
+    expect(result.isInPartialStart).toBe(false);
+    expect(result.modifiedBuffer).toBe(buffer);
+  });
 });
diff --git a/core/tools/systemMessageTools/toolCodeblocks/interceptSystemToolCalls.vitest.ts b/core/tools/systemMessageTools/toolCodeblocks/interceptSystemToolCalls.vitest.ts
index a636cc6a9da..b03e0309ce4 100644
--- a/core/tools/systemMessageTools/toolCodeblocks/interceptSystemToolCalls.vitest.ts
+++ b/core/tools/systemMessageTools/toolCodeblocks/interceptSystemToolCalls.vitest.ts
@@ -179,9 +179,8 @@ describe("interceptSystemToolCalls", () => {
     ).toBe("}");
   });
 
-  it("processes tool_name without codeblock format", async () => {
+  it("processes tool_name without codeblock format at assistant output start", async () => {
     const messages: ChatMessage[][] = [
-      [{ role: "assistant", content: "I'll help you with that.\n" }],
       [{ role: "assistant", content: "TOOL_NAME: test_tool\n" }],
       [{ role: "assistant", content: "BEGIN_ARG: arg1\n" }],
       [{ role: "assistant", content: "value1\n" }],
@@ -194,30 +193,7 @@ describe("interceptSystemToolCalls", () => {
       framework,
     );
 
-    // First chunk should be normal text
     let result = await generator.next();
-    expect(result.value).toEqual([
-      {
-        role: "assistant",
-        content: [{ type: "text", text: "I'll help you with that." }],
-      },
-    ]);
-
-    result = await generator.next();
-    expect(result.value).toEqual([
-      {
-        role: "assistant",
-        content: [
-          {
-            type: "text",
-            text: "\n",
-          },
-        ],
-      },
-    ]);
-
-    // The system should detect the tool_name format and convert it
-    result = await generator.next();
     expect(
       (result.value as AssistantChatMessage[])[0].toolCalls?.[0].function?.name,
     ).toBe("test_tool");
@@ -242,6 +218,43 @@ describe("interceptSystemToolCalls", () => {
     ).toBe("}");
   });
 
+  it("does not intercept quoted tool syntax in explanatory text", async () => {
+    const messages: ChatMessage[][] = [
+      [{ role: "assistant", content: "Here is the syntax:\n" }],
+      [{ role: "assistant", content: "TOOL_NAME: read_file\n" }],
+      [{ role: "assistant", content: "BEGIN_ARG: filepath\n" }],
+      [{ role: "assistant", content: "path/to/the_file.txt\n" }],
+      [{ role: "assistant", content: "END_ARG\n" }],
+    ];
+
+    const generator = interceptSystemToolCalls(
+      createAsyncGenerator(messages),
+      abortController,
+      framework,
+    );
+
+    const outputChunks: string[] = [];
+    while (true) {
+      const result = await generator.next();
+      if (result.done || !result.value) {
+        break;
+      }
+
+      const chunkText = (
+        (result.value as AssistantChatMessage[])[0].content as {
+          type: "text";
+          text: string;
+        }[]
+      )[0].text;
+      outputChunks.push(chunkText);
+      expect((result.value as AssistantChatMessage[])[0].toolCalls).toBeFalsy();
+    }
+
+    expect(outputChunks.join("")).toBe(
+      "Here is the syntax:\nTOOL_NAME: read_file\nBEGIN_ARG: filepath\npath/to/the_file.txt\nEND_ARG\n",
+    );
+  });
+
   it("ignores content after a tool call", async () => {
     const messages: ChatMessage[][] = [
       [{ role: "assistant", content: "```tool\n" }],