Braintrust Tracing

Braintrust Tracing for Claude Code

Comprehensive guide to tracing Claude Code sessions in Braintrust, including sub-agent correlation.

Architecture Overview

                         PARENT SESSION                    +---------------------+                    |  SessionStart       |                    |  (creates root)     |                    +----------+----------+                               |                    +----------v----------+                    |  UserPromptSubmit   |                    |  (creates Turn)     |                    +----------+----------+                               |          +--------------------+--------------------+          |                    |                    |+---------v--------+  +--------v--------+  +--------v--------+| PostToolUse      |  | PostToolUse     |  | PreToolUse      || (Read span)      |  | (Edit span)     |  | (Task - inject) |+------------------+  +-----------------+  +--------+--------+                                                    |                                         +----------v----------+                                         |   SUB-AGENT         |                                         |   SessionStart      |                                         |   (NEW root_span_id)|                                         +----------+----------+                                                    |                                         +----------v----------+                                         |   SubagentStop      |                                         |   (has session_id)  |                                         +---------------------+

Hook Event Flow

Trace Hierarchy

Session (task span) - root_span_id = session_id|+-- Turn 1 (task span)|   ||   +-- claude-sonnet (llm span) - model call with tool_use|   +-- Read (tool span)|   +-- Edit (tool span)|   +-- claude-sonnet (llm span) - response after tools|+-- Turn 2 (task span)|   ||   +-- claude-sonnet (llm span)|   +-- Task (tool span) -----> [Sub-agent session - SEPARATE trace]|   +-- claude-sonnet (llm span)|+-- Turn 3 ...

Sub-Agent Tracing: What Works and What Doesn't

What Doesn't Work

SessionStart doesn't receive the Task prompt.

We tried injecting trace context into Task prompts via PreToolUse:

# PreToolUse hook injects:[BRAINTRUST_TRACE_CONTEXT]{"root_span_id": "abc", "parent_span_id": "xyz", "project_id": "123"}[/BRAINTRUST_TRACE_CONTEXT]

But SessionStart only receives session metadata, not the modified prompt. The injected context is lost.

What DOES Work

Task spans in parent session contain everything:

• agentId - identifier for the sub-agent run
• totalTokens, totalToolUseCount - metrics
• content - full agent response/summary
• tool_input.prompt - original task prompt
• tool_input.subagent_type - agent type (e.g., "oracle")

SubagentStop hook receives the sub-agent's session_id:

• This equals the sub-agent's orphaned trace root_span_id
• Allows correlation between parent Task span and child trace

The Correlation Pattern

Current state: Sub-agents create orphaned traces (new root_span_id).

Correlation method:

1. Query parent session's Task spans for agent metadata
2. Match agentId or timing with orphaned traces
3. Sub-agent's session_id = its trace's root_span_id

Future solution (not yet implemented):

SubagentStop fires -> writes session_id to temp filePostToolUse (Task) -> reads temp file -> adds child_session_id to Task span metadata

This would link: Task.agentId + Task.child_session_id -> orphaned trace root_span_id

State Management

Per-Session State Files

~/.claude/state/braintrust_sessions/  {session_id}.json       # Per-session state

Each session file contains:

{  "root_span_id": "abc-123",  "project_id": "proj-456",  "turn_count": 5,  "tool_count": 23,  "current_turn_span_id": "turn-789",  "current_turn_start": 1703456789,  "started": "2025-12-24T10:00:00.000Z",  "is_subagent": false}

Global State

~/.claude/state/braintrust_global.json   # Cached project_id~/.claude/state/braintrust_hook.log      # Debug log

Debugging Commands

Check if Tracing is Active

# View hook logs in real-timetail -f ~/.claude/state/braintrust_hook.log
# Check if session has statecat ~/.claude/state/braintrust_sessions/*.json | jq -s '.'
# Verify environmentecho "TRACE_TO_BRAINTRUST=$TRACE_TO_BRAINTRUST"echo "BRAINTRUST_API_KEY=${BRAINTRUST_API_KEY:+set}"

Query Braintrust Directly

# List recent sessionsuv run python -m runtime.harness scripts/braintrust_analyze.py --sessions 5
# Analyze last sessionuv run python -m runtime.harness scripts/braintrust_analyze.py --last-session
# Replay specific sessionuv run python -m runtime.harness scripts/braintrust_analyze.py --replay <session-id>
# Find sub-agent traces (orphaned roots)uv run python -m runtime.harness scripts/braintrust_analyze.py --agent-stats

Debug Hook Execution

# Enable verbose loggingexport BRAINTRUST_CC_DEBUG=true
# Test hooks manuallyecho '{"session_id":"test-123","type":"resume"}' | \  bash "$CLAUDE_PROJECT_DIR/.claude/plugins/braintrust-tracing/hooks/session_start.sh"
# Test PreToolUse (Task injection)echo '{"session_id":"test-123","tool_name":"Task","tool_input":{"prompt":"test"}}' | \  bash "$CLAUDE_PROJECT_DIR/.claude/plugins/braintrust-tracing/hooks/pre_tool_use.sh"

Troubleshooting Checklist

1. No traces appearing:

   • Check TRACE_TO_BRAINTRUST=true in .claude/settings.local.json
   • Verify API key: echo $BRAINTRUST_API_KEY
   • Check logs: tail -20 ~/.claude/state/braintrust_hook.log

2. Sub-agents not linking:

   • This is expected - sub-agents create orphaned traces
   • Use --agent-stats to find agent activity
   • Correlate via timing or agentId in parent Task span

3. Missing spans:

   • Check current_turn_span_id in session state
   • Ensure Stop hook runs (turn finalization)
   • Look for "Failed to create" errors in log

4. State corruption:

   • Remove session state: rm ~/.claude/state/braintrust_sessions/*.json
   • Clear global cache: rm ~/.claude/state/braintrust_global.json

Key Files

Environment Variables

Session Learnings

What We Learned About Sub-Agent Tracing (Dec 2025)

Attempted: Inject trace context via PreToolUse into Task prompts.

Result: Failed - SessionStart only receives session metadata, not the prompt.

Discovery: Task spans already contain rich sub-agent data:

• metadata.agent_type - agent type from subagent_type
• metadata.skill_name - skill from Skill tool
• tool_input - full prompt sent to agent
• tool_output - agent response

Current correlation path:

1. Parent session Task span has agentId and timing
2. Sub-agent creates orphaned trace with root_span_id = session_id
3. SubagentStop provides the sub-agent's session_id
4. Manual correlation: match timing or use session_id link

Future work: Write child_session_id to Task span metadata from PostToolUse after SubagentStop.

What We Learned About Sub-Agent Correlation

The Problem

• Sub-agents spawned via Task tool create orphaned Braintrust traces
• Parent session has Task spans with agentId, sub-agent has separate session_id
• No built-in link between them

What DOESN'T Work

1. Prompt injection via PreToolUse

SessionStart hook only receives session metadata (session_id, type, cwd), NOT the prompt. Injected trace context is never seen.

The hook receives:

{  "session_id": "...",  "type": "start|resume|compact|clear",  "cwd": "...",  "env": {...}}

No prompt field exists - context injection is impossible at SessionStart.

2. SubagentStop → PostToolUse file handoff

Race condition. These are independent async hooks with no timing guarantees:

• SubagentStop fires when sub-agent session ends
• PostToolUse (Task) fires when Task tool completes
• No ordering guarantee between them
• Writing to a correlation file creates a race

3. PreToolUse correlation files

SessionStart can't access the task_span_id because it has no context about which Task spawned it. PreToolUse modifies prompts but doesn't create a reliably accessible state file that SessionStart can find.

What DOES Work

Post-hoc matching for dataset building:

Parent session Task spans contain:

• agentId - identifier for the sub-agent run
• totalTokens, totalToolUseCount - aggregated metrics
• content - full agent response/summary
• tool_input.prompt - original task prompt
• tool_input.subagent_type - agent type (e.g., "oracle")
• Start/end timestamps

Sub-agent sessions contain:

• session_id (equals orphaned trace root_span_id)
• Start/end timestamps
• All internal spans and tool calls

Correlation strategy:

1. Export parent session traces (query parent root_span_id)
2. Export sub-agent traces (query all sessions created within parent's time window)
3. Match by:
   • Timing: Task span end ≈ sub-agent session end
   • Metadata: subagent_type from Task prompt
   • IDs: SubagentStop hook provides session_id (can be captured and logged)

Architecture Insight

SessionStart input is intentionally minimal - it contains no prompt or tool context:

interface SessionStartInput {  session_id: string;  type: "start" | "resume" | "compact" | "clear";  cwd: string;  env: { [key: string]: string };  // NO: prompt, tool_context, task_span_id, parent_span_id}

This design boundary prevents real-time correlation at hook time.

Recommendation

For building agent run datasets with sub-agent correlation:

1. In-session logging: Capture SubagentStop session_id in logs or state
2. Post-session export: Query Braintrust API for parent and sub-agent traces
3. Offline correlation: Match traces by timing and metadata in a script
4. Don't try real-time linking: Hooks don't have necessary context

Example script pattern:

# 1. Export parent sessionbraintrust_analyze.py --replay <parent-session-id> > parent_traces.json
# 2. Query for orphaned sub-agent traces (those created during parent's time window)braintrust_analyze.py --agent-stats > all_agent_traces.json
# 3. Correlate in Python:#    - Parent Task spans -> agentId, timestamps, subagent_type#    - Orphaned traces -> root_span_id, timestamps#    - Match by timing and type

This approach is reliable, testable, and doesn't require hooks to maintain implicit state.