fix(langchain): record tool span input as structured data

[GeneralistDev]

💡 Motivation and Context

LangChain's BaseTool.run/arun hand the tool input to on_tool_start twice: positionally as str(tool_input), and as the original dict under the inputs keyword (langchain_core/tools/base.py lines 838/846 and 949/957). CallbackHandler.on_tool_start stored the positional one, so a dict tool input landed in $ai_input_state as a Python repr:

{'query': 'SELECT 1', 'truncate': True}

Single quotes, True instead of true — no JSON parser reads that. In ClickHouse JSONExtract* returns empty against the input_state column, so anything consuming tool spans has to fall back to substring matching on the raw text, and any analysis that assumed valid JSON silently returns zero rows rather than failing loudly.

on_chain_start already stores the real inputs dict, so chain spans were fine and only tool spans were affected — which made the gap easy to miss.

This prefers the structured inputs dict when LangChain supplies one. Tools invoked with a plain string have no inputs and keep recording the string unchanged.

💚 How did you test it?

Two unit tests driving on_tool_start/on_tool_end the way BaseTool.arun does — one asserting a dict input is recorded as a dict, one asserting a string input still records the string.

uv run --extra test python -m pytest posthog/test/ai/langchain/test_callbacks.py — 77 passed, 7 failed. The same 7 fail on an unmodified main in this environment (they need OPENAI_API_KEY/ANTHROPIC_API_KEY and network), so no regressions from this change.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Found while investigating whether MCP and Max agents lean on raw SQL where a typed query runner would do. Measuring that needs the query text off the tool span, and every JSONExtract against input_state came back empty for Max's execute_sql spans while the MCP path's spans parsed fine — which is what led back to this handler.

Root-causing went one layer at a time: ruled out the server-side truncation hook in the main repo (it only caps strings that are already strings, and uses json.dumps on its one stringifying path), then the SDK's capture plumbing (passes properties through untouched), before landing on the positional-vs-keyword argument in on_tool_start. The str() itself happens in langchain-core and can't be changed here; taking the inputs kwarg it already passes alongside is the fix on our side.

Considered normalizing the value at query time instead and rejected it — that leaves every existing and future consumer parsing Python repr, and the correct value is already being handed to us.

Note this changes the shape of $ai_input_state for tool spans from a string to an object. That matches chain spans and is the intended fix, but anyone who built around the repr string will see the type change.

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Created with PostHog from a Slack thread

[github-actions]

posthog-python Compliance Report

Date: 2026-08-31 13:06:42 UTC
Duration: 256404ms

✅ All Tests Passed!

111/111 tests passed

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Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	516ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	510ms
Required Headers.Has Authorization Bearer Header	✅	510ms
Required Headers.Has Content Type Json	✅	510ms
Required Headers.Has Posthog Sdk Info Format	✅	510ms
Required Headers.Has Posthog Attempt Header	✅	510ms
Required Headers.Has Posthog Request Id	✅	510ms
Required Headers.Has Posthog Request Timestamp	✅	509ms
Required Headers.Has User Agent	✅	510ms
Body Format.Body Has Created At And Batch	✅	511ms
Body Format.No Api Key In Body	✅	511ms
Body Format.No Sent At In Body	✅	510ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	510ms
Event Format.Event Timestamp Is Rfc3339	✅	509ms
Event Format.Distinct Id Is String	✅	511ms
Event Format.Distinct Id At Root Not Properties	✅	510ms
Event Format.Custom Properties Preserved	✅	510ms
Event Format.Set Properties Preserved	✅	509ms
Event Format.Set Once Properties Preserved	✅	510ms
Event Format.Groups Properties Preserved	✅	510ms
Event Format.Sdk Generates Uuid If Not Provided	✅	510ms
Event Format.Event Has Required Root Fields Batch	✅	513ms
Event Format.Event Uuid Is Valid Batch	✅	513ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	514ms
Event Format.Distinct Id Is String Batch	✅	513ms
Event Format.Distinct Id At Root Not Properties Batch	✅	513ms
Event Format.Custom Properties Preserved Batch	✅	513ms
Event Format.Set Properties Preserved Batch	✅	513ms
Event Format.Set Once Properties Preserved Batch	✅	513ms
Event Format.Groups Properties Preserved Batch	✅	513ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	514ms
Batch Behavior.Multiple Events In Single Batch	✅	517ms
Batch Behavior.Batch Envelope Smoke	✅	515ms
Batch Behavior.Flush With No Events Sends Nothing	✅	506ms
Batch Behavior.Flush At Triggers Batch	✅	1011ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	510ms
Deduplication.Generates Unique Uuids	✅	517ms
Deduplication.Different Events Same Content Different Uuids	✅	512ms
Deduplication.Preserves Uuid On Retry	✅	6518ms
Deduplication.Preserves Timestamp On Retry	✅	6517ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6522ms
Deduplication.No Duplicate Events In Batch	✅	517ms
Header Behavior On Retry.Attempt Header Starts At One	✅	510ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13526ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6515ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3019ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6519ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	510ms
Response Format Validation.Success Response Has Ok For Each Event	✅	513ms
Response Format Validation.Success No Retry After When All Ok	✅	512ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1516ms
Response Format Validation.Success No Retry After When Drop Only	✅	512ms
Response Format Validation.Response Echoes Request Id	✅	510ms
Retry Behavior.Retries On 408	✅	6519ms
Retry Behavior.Retries On 500	✅	6519ms
Retry Behavior.Retries On 503	✅	8522ms
Retry Behavior.Retries On 504	✅	6519ms
Retry Behavior.Retryable Errors Have Retry After	✅	3515ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11522ms
Retry Behavior.Does Not Retry On 400	✅	2513ms
Retry Behavior.Does Not Retry On 401	✅	2513ms
Retry Behavior.Does Not Retry On 402	✅	2513ms
Retry Behavior.Does Not Retry On 413	✅	2515ms
Retry Behavior.Does Not Retry On 415	✅	2512ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2513ms
Retry Behavior.Implements Backoff	✅	22536ms
Retry Behavior.Max Retries Respected	✅	22536ms
Partial Batch Handling.Handles 200 Full Success	✅	2512ms
Partial Batch Handling.Handles 200 With All Ok	✅	3519ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3516ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3515ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6522ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6521ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6524ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6517ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6521ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6522ms
Partial Batch Handling.Respects Retry After On Partial	✅	8517ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3516ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3518ms
Compression.Sends Gzip Content Encoding	✅	511ms
Compression.No Content Encoding When Disabled	✅	511ms
Compression.Compressed Body Is Decompressible	✅	510ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2513ms
Event Options.Cookieless Mode Override	✅	510ms
Event Options.Disable Skew Correction Override	✅	510ms
Event Options.Process Person Profile Override	✅	510ms
Event Options.Product Tour Id Override	✅	510ms
Event Options.Unset Options Omitted	✅	509ms
Event Options.Options Override In Batch	✅	513ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	511ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	510ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	12ms
Request Payload.Flags Request Uses V2 Query Param	✅	10ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	9ms
Request Payload.Flags Request Omits Authorization Header	✅	10ms
Request Payload.Token In Flags Body Matches Init	✅	10ms
Request Payload.Groups Round Trip	✅	9ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	11ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	510ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10ms
Retry Behavior.Retries Flags On 502	✅	313ms
Retry Behavior.Retries Flags On 504	✅	314ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	513ms