Session Investigator

Diagnose fast-agent session issues by examining session and history files.

Session Directory Structure

Sessions are stored in .fast-agent/sessions/<session-id>/ :

2601181023-Kob2h3/ ├── session.json # Session metadata ├── history_<agent>.json # Current agent history
└── history_<agent>_previous.json # Previous save (rotation backup)

Session IDs encode creation time: YYMMDDHHMM-<random> (e.g., 2601181023 = 2026-01-18 10:23).

Key Files

session.json

{ "name": "2601181023-Kob2h3", "created_at": "2026-01-18T10:23:24.116526", "last_activity": "2026-01-18T10:39:42.873467", "history_files": ["history_dev_previous.json", "history_dev.json"], "metadata": { "agent_name": "dev", "first_user_preview": "is it possible to override..." } }

history_.json

{ "messages": [ { "role": "user|assistant", "content": [{"type": "text", "text": "..."}], "tool_calls": {"<id>": {"method": "tools/call", "params": {"name": "...", "arguments": {}}}}, "tool_results": {"<id>": {"content": [...], "isError": false}}, "channels": { "fast-agent-timing": [{"type": "text", "text": "{"start_time": ..., "end_time": ..., "duration_ms": ...}"}], "fast-agent-tool-timing": [{"type": "text", "text": "{"<tool_id>": {"timing_ms": ..., "transport_channel": ...}}"}], "reasoning": [{"type": "text", "text": "..."}] }, "stop_reason": "endTurn|toolUse|error", "is_template": false } ] }

Investigation Commands

Basic inspection

Message count

jq '.messages | length' history_dev.json

Last N messages overview

jq '.messages[-5:] | .[] | {role, stop_reason, has_tool_calls: (.tool_calls != null), has_tool_results: (.tool_results != null)}' history_dev.json

View specific message

jq '.messages[227]' history_dev.json

Tool call correlation

Tool calls and results are linked by correlation ID. Valid pattern: assistant with tool_calls → user with matching tool_results .

Check tool call/result pairing

jq '.messages[-10:] | to_entries | .[] | { index: .key, role: .value.role, tool_calls: (if .value.tool_calls then (.value.tool_calls | keys) else [] end), tool_results: (if .value.tool_results then (.value.tool_results | keys) else [] end) }' history_dev.json

Find specific tool calls

Find all calls to a specific tool

jq '.messages | to_entries | .[] | select(.value.tool_calls != null) | select(.value.tool_calls | to_entries | .[0].value.params.name == "agent__ripgrep_search") | {index: .key, timing: (.value.channels."fast-agent-timing"[0].text)}' history_dev.json

Session Statistics

LLM Call Stats

Total LLM time and call count

jq '[.messages[] | select(.role == "assistant") | select(.channels."fast-agent-timing") | .channels."fast-agent-timing"[0].text | fromjson | .duration_ms] | {count: length, total_ms: add, avg_ms: (add/length), max_ms: max, min_ms: min}' history_dev.json

LLM calls sorted by duration (slowest first)

jq '[.messages | to_entries | .[] | select(.value.role == "assistant") | select(.value.channels."fast-agent-timing") | {index: .key, duration_ms: (.value.channels."fast-agent-timing"[0].text | fromjson | .duration_ms)}] | sort_by(-.duration_ms) | .[0:10]' history_dev.json

Tool Execution Stats

All tool timings aggregated

jq '[.messages[] | select(.channels."fast-agent-tool-timing") | .channels."fast-agent-tool-timing"[0].text | fromjson | to_entries | .[].value.timing_ms] | {count: length, total_ms: add, avg_ms: (add/length), max_ms: max, min_ms: min}' history_dev.json

Tool calls by name with timing

jq '[.messages | to_entries | .[] | select(.value.tool_calls) | (.value.tool_calls | to_entries | .[0]) as $tc | {index: .key, tool: $tc.value.params.name, llm_ms: (.value.channels."fast-agent-timing"[0].text | fromjson | .duration_ms)}] | group_by(.tool) | map({tool: .[0].tool, count: length, total_llm_ms: (map(.llm_ms) | add)}) | sort_by(-.count)' history_dev.json

Session Timeline

Session duration from first to last timing

jq '.messages | [ (map(select(.channels."fast-agent-timing")) | first | .channels."fast-agent-timing"[0].text | fromjson | .start_time), (map(select(.channels."fast-agent-timing")) | last | .channels."fast-agent-timing"[0].text | fromjson | .end_time) ] | {start: .[0], end: .[1], duration_sec: ((.[1] - .[0]) | round)}' history_dev.json

Message rate over time (messages per minute estimate)

jq '{ messages: (.messages | length), llm_calls: [.messages[] | select(.role == "assistant" and .channels."fast-agent-timing")] | length, total_llm_ms: [.messages[] | select(.channels."fast-agent-timing") | .channels."fast-agent-timing"[0].text | fromjson | .duration_ms] | add, total_tool_ms: [.messages[] | select(.channels."fast-agent-tool-timing") | .channels."fast-agent-tool-timing"[0].text | fromjson | to_entries | .[].value.timing_ms] | add } | . + {llm_sec: (.total_llm_ms/1000), tool_sec: ((.total_tool_ms//0)/1000)}' history_dev.json

Sub-agent Stats

Sub-agent calls (tools starting with "agent__")

jq '[.messages | to_entries | .[] | select(.value.tool_calls) | (.value.tool_calls | to_entries | .[0]) as $tc | select($tc.value.params.name | startswith("agent__")) | {index: .key, agent: $tc.value.params.name, llm_ms: (.value.channels."fast-agent-timing"[0].text | fromjson | .duration_ms)}] | group_by(.agent) | map({agent: .[0].agent, calls: length, total_ms: (map(.llm_ms) | add), avg_ms: ((map(.llm_ms) | add) / length)})' history_dev.json

Common Failure Patterns

Unanswered Tool Call

Symptom: API error "No tool output found for function call"

Pattern: History ends with assistant message having tool_calls and stop_reason: "toolUse" , followed by user message WITHOUT matching tool_results .

Check last message for pending tool call

jq '.messages[-1] | {role, has_tool_calls: (.tool_calls != null), stop_reason}' history_dev.json

Cause: Session interrupted mid-tool-loop, then resumed with new user input before tool completed.

Fix: Truncate history to last valid tool result:

Find last user message with tool_results

jq '.messages | to_entries | map(select(.value.role == "user" and .value.tool_results != null)) | last | .key' history_dev.json

Truncate (keep messages 0 to N inclusive, so use N+1)

jq '.messages = .messages[0:227]' history_dev.json > /tmp/fixed.json && mv /tmp/fixed.json history_dev.json

Duplicate User Messages

Pattern: Two consecutive user messages before assistant response.

Cause: Often from before_llm_call hooks appending instructions. Check agent card's tool_hooks configuration.

Sub-agent Trace Correlation

Sub-agent traces are saved as <agent_name>-<timestamp>.json in the working directory.

List traces around session time

ls -la ripgrep_search2026-01-18-10-3.json

Correlate via timing - match monotonic clock values

jq '.messages[-1].channels."fast-agent-timing"[0].text' ripgrep_search*.json

Compare start_time /end_time values between main session and sub-agent traces to correlate which sub-agent call corresponds to which main session tool call.

Log File

Check fastagent.jsonl for errors during the session timeframe:

Filter by timestamp range

cat fastagent.jsonl | while read line; do ts=$(echo "$line" | jq -r '.timestamp // empty' 2>/dev/null) if [[ "$ts" > "2026-01-18T10:20" && "$ts" < "2026-01-18T10:45" ]]; then echo "$line" | jq -c '{timestamp, level, message}' fi done