fix(langchain): properly retrieve structured output from thinking block

[christian-bromann]

Fixes an issue where responseFormat with providerStrategy returns undefined when used with thinking/reasoning models (e.g., Claude with thinking enabled, OpenAI o1/o3).

fixes #9421

Problem

When using thinking models, the AI message content is returned as an array of content blocks (containing thinking blocks and text blocks), rather than a simple string:

// Thinking model response structure
content: [
  { type: "thinking", thinking: "..." },
  { type: "text", text: '{"shapeName": "circle", ...}' }  // ← Structured output is here
]

The ProviderStrategy.parse() method only handled string content, causing it to return undefined for all thinking model responses:

// BEFORE (broken)
if (typeof response.content !== "string" || response.content === "") {
  return; // Returns undefined for array content!
}

Solution

Updated ProviderStrategy.parse() to handle both string content and array content by extracting text from text blocks:

// AFTER (fixed)
let textContent: string | undefined;

if (typeof response.content === "string") {
  textContent = response.content;
} else if (Array.isArray(response.content)) {
  // For thinking models, extract text from text blocks
  for (const block of response.content) {
    if (block.type === "text" && typeof block.text === "string") {
      textContent = block.text;
      break;
    }
  }
}

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🦋 Changeset detected

Latest commit: e270990

The changes in this PR will be included in the next version bump.

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Name	Type
langchain	Patch

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