fix(langchain): keep tool call / AIMessage pairings when summarizing

[christian-bromann]

Port of langchain-ai/langchain#34609

Problem

When using agents with tools (like file reading, web search, etc.), the conversation looks like this:

[User]     "Read these 10 files and summarize them"
[AI]       "I'll read all 10 files" + [tool_call: read_file x 10]
[Tool]     "Contents of file1.txt..."
[Tool]     "Contents of file2.txt..."
...

When the conversation gets too long, SummarizationMiddleware kicks in to compress older messages. The problem was:

If you asked to keep the last 6 messages, you'd get:

[Summary]  "Here's what happened before..."
[Tool]     "Contents of file5.txt..."
[Tool]     "Contents of file6.txt..."
...

The AI's original request to read the files ([AI] message with tool_calls) was summarized away, but the tool responses remained. This caused the error:

Error code: 400 - "No tool call found for function call output with call_id..."

Many APIs require that every tool response has a matching tool request. Without the AI message, the tool responses are "orphaned."

The Fix

Now when the cutoff lands on tool messages, we move backward to include the AI message that requested those tools:

[Summary]  "Here's what happened before..."
[AI]       "I'll read all 10 files" + [tool_call: read_file x 10]
[Tool]     "Contents of file1.txt..."
[Tool]     "Contents of file2.txt..."
... (all tool responses preserved with their AIMessage)

The AI message is preserved along with its tool responses, keeping them paired together.

Changes

• Added findSafeCutoffPoint() function that searches backward for matching AIMessage when cutoff lands on ToolMessage
• Updated findTokenBasedCutoff() and findSafeCutoff() to use the new function
• Added tests for AI/Tool pair preservation including parallel tool calls scenario

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Latest commit: 116f10b

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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