hermes-agent/website/docs/developer-guide/session-storage.md

Session Storage

Hermes Agent uses a SQLite database (~/.hermes/state.db) to persist session metadata, full message history, and model configuration across CLI and gateway sessions. This replaces the earlier per-session JSONL file approach.

Source file: hermes_state.py

Architecture Overview

~/.hermes/state.db (SQLite, WAL mode)
├── sessions          — Session metadata, token counts, billing
├── messages          — Full message history per session
├── messages_fts      — FTS5 virtual table for full-text search
└── schema_version    — Single-row table tracking migration state

Key design decisions:

• WAL mode for concurrent readers + one writer (gateway multi-platform)
• FTS5 virtual table for fast text search across all session messages
• Session lineage via parent_session_id chains (compression-triggered splits)
• Source tagging (cli, telegram, discord, etc.) for platform filtering
• Batch runner and RL trajectories are NOT stored here (separate systems)

SQLite Schema

Sessions Table

CREATE TABLE IF NOT EXISTS sessions (
    id TEXT PRIMARY KEY,
    source TEXT NOT NULL,
    user_id TEXT,
    model TEXT,
    model_config TEXT,
    system_prompt TEXT,
    parent_session_id TEXT,
    started_at REAL NOT NULL,
    ended_at REAL,
    end_reason TEXT,
    message_count INTEGER DEFAULT 0,
    tool_call_count INTEGER DEFAULT 0,
    input_tokens INTEGER DEFAULT 0,
    output_tokens INTEGER DEFAULT 0,
    cache_read_tokens INTEGER DEFAULT 0,
    cache_write_tokens INTEGER DEFAULT 0,
    reasoning_tokens INTEGER DEFAULT 0,
    billing_provider TEXT,
    billing_base_url TEXT,
    billing_mode TEXT,
    estimated_cost_usd REAL,
    actual_cost_usd REAL,
    cost_status TEXT,
    cost_source TEXT,
    pricing_version TEXT,
    title TEXT,
    FOREIGN KEY (parent_session_id) REFERENCES sessions(id)
);

CREATE INDEX IF NOT EXISTS idx_sessions_source ON sessions(source);
CREATE INDEX IF NOT EXISTS idx_sessions_parent ON sessions(parent_session_id);
CREATE INDEX IF NOT EXISTS idx_sessions_started ON sessions(started_at DESC);
CREATE UNIQUE INDEX IF NOT EXISTS idx_sessions_title_unique
    ON sessions(title) WHERE title IS NOT NULL;

Messages Table

CREATE TABLE IF NOT EXISTS messages (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    session_id TEXT NOT NULL REFERENCES sessions(id),
    role TEXT NOT NULL,
    content TEXT,
    tool_call_id TEXT,
    tool_calls TEXT,
    tool_name TEXT,
    timestamp REAL NOT NULL,
    token_count INTEGER,
    finish_reason TEXT,
    reasoning TEXT,
    reasoning_details TEXT,
    codex_reasoning_items TEXT
);

CREATE INDEX IF NOT EXISTS idx_messages_session ON messages(session_id, timestamp);

Notes:

• tool_calls is stored as a JSON string (serialized list of tool call objects)
• reasoning_details and codex_reasoning_items are stored as JSON strings
• reasoning stores the raw reasoning text for providers that expose it
• Timestamps are Unix epoch floats (time.time())

FTS5 Full-Text Search

CREATE VIRTUAL TABLE IF NOT EXISTS messages_fts USING fts5(
    content,
    content=messages,
    content_rowid=id
);

The FTS5 table is kept in sync via three triggers that fire on INSERT, UPDATE, and DELETE of the messages table:

CREATE TRIGGER IF NOT EXISTS messages_fts_insert AFTER INSERT ON messages BEGIN
    INSERT INTO messages_fts(rowid, content) VALUES (new.id, new.content);
END;

CREATE TRIGGER IF NOT EXISTS messages_fts_delete AFTER DELETE ON messages BEGIN
    INSERT INTO messages_fts(messages_fts, rowid, content)
        VALUES('delete', old.id, old.content);
END;

CREATE TRIGGER IF NOT EXISTS messages_fts_update AFTER UPDATE ON messages BEGIN
    INSERT INTO messages_fts(messages_fts, rowid, content)
        VALUES('delete', old.id, old.content);
    INSERT INTO messages_fts(rowid, content) VALUES (new.id, new.content);
END;

Schema Version and Migrations

Current schema version: 6

The schema_version table stores a single integer. On initialization, _init_schema() checks the current version and applies migrations sequentially:

Version	Change
1	Initial schema (sessions, messages, FTS5)
2	Add finish_reason column to messages
3	Add title column to sessions
4	Add unique index on title (NULLs allowed, non-NULL must be unique)
5	Add billing columns: cache_read_tokens, cache_write_tokens, reasoning_tokens, billing_provider, billing_base_url, billing_mode, estimated_cost_usd, actual_cost_usd, cost_status, cost_source, pricing_version
6	Add reasoning columns to messages: reasoning, reasoning_details, codex_reasoning_items

Each migration uses ALTER TABLE ADD COLUMN wrapped in try/except to handle the column-already-exists case (idempotent). The version number is bumped after each successful migration block.

Write Contention Handling

Multiple hermes processes (gateway + CLI sessions + worktree agents) share one state.db. The SessionDB class handles write contention with:

• Short SQLite timeout (1 second) instead of the default 30s
• Application-level retry with random jitter (20-150ms, up to 15 retries)
• BEGIN IMMEDIATE transactions to surface lock contention at transaction start
• Periodic WAL checkpoints every 50 successful writes (PASSIVE mode)

This avoids the "convoy effect" where SQLite's deterministic internal backoff causes all competing writers to retry at the same intervals.

_WRITE_MAX_RETRIES = 15
_WRITE_RETRY_MIN_S = 0.020   # 20ms
_WRITE_RETRY_MAX_S = 0.150   # 150ms
_CHECKPOINT_EVERY_N_WRITES = 50

Common Operations

Initialize

from hermes_state import SessionDB

db = SessionDB()                           # Default: ~/.hermes/state.db
db = SessionDB(db_path=Path("/tmp/test.db"))  # Custom path

Create and Manage Sessions

# Create a new session
db.create_session(
    session_id="sess_abc123",
    source="cli",
    model="anthropic/claude-sonnet-4.6",
    user_id="user_1",
    parent_session_id=None,  # or previous session ID for lineage
)

# End a session
db.end_session("sess_abc123", end_reason="user_exit")

# Reopen a session (clear ended_at/end_reason)
db.reopen_session("sess_abc123")

Store Messages

msg_id = db.append_message(
    session_id="sess_abc123",
    role="assistant",
    content="Here's the answer...",
    tool_calls=[{"id": "call_1", "function": {"name": "terminal", "arguments": "{}"}}],
    token_count=150,
    finish_reason="stop",
    reasoning="Let me think about this...",
)

Retrieve Messages

# Raw messages with all metadata
messages = db.get_messages("sess_abc123")

# OpenAI conversation format (for API replay)
conversation = db.get_messages_as_conversation("sess_abc123")
# Returns: [{"role": "user", "content": "..."}, {"role": "assistant", ...}]

Session Titles

# Set a title (must be unique among non-NULL titles)
db.set_session_title("sess_abc123", "Fix Docker Build")

# Resolve by title (returns most recent in lineage)
session_id = db.resolve_session_by_title("Fix Docker Build")

# Auto-generate next title in lineage
next_title = db.get_next_title_in_lineage("Fix Docker Build")
# Returns: "Fix Docker Build #2"

Full-Text Search

The search_messages() method supports FTS5 query syntax with automatic sanitization of user input.

Basic Search

results = db.search_messages("docker deployment")

FTS5 Query Syntax

Syntax	Example	Meaning
Keywords	docker deployment	Both terms (implicit AND)
Quoted phrase	"exact phrase"	Exact phrase match
Boolean OR	docker OR kubernetes	Either term
Boolean NOT	python NOT java	Exclude term
Prefix	deploy*	Prefix match

Filtered Search

# Search only CLI sessions
results = db.search_messages("error", source_filter=["cli"])

# Exclude gateway sessions
results = db.search_messages("bug", exclude_sources=["telegram", "discord"])

# Search only user messages
results = db.search_messages("help", role_filter=["user"])

Search Results Format

Each result includes:

• id, session_id, role, timestamp
• snippet — FTS5-generated snippet with >>>match<<< markers
• context — 1 message before and after the match (content truncated to 200 chars)
• source, model, session_started — from the parent session

The _sanitize_fts5_query() method handles edge cases:

• Strips unmatched quotes and special characters
• Wraps hyphenated terms in quotes (chat-send → "chat-send")
• Removes dangling boolean operators (hello AND → hello)

Session Lineage

Sessions can form chains via parent_session_id. This happens when context compression triggers a session split in the gateway.

Query: Find Session Lineage

-- Find all ancestors of a session
WITH RECURSIVE lineage AS (
    SELECT * FROM sessions WHERE id = ?
    UNION ALL
    SELECT s.* FROM sessions s
    JOIN lineage l ON s.id = l.parent_session_id
)
SELECT id, title, started_at, parent_session_id FROM lineage;

-- Find all descendants of a session
WITH RECURSIVE descendants AS (
    SELECT * FROM sessions WHERE id = ?
    UNION ALL
    SELECT s.* FROM sessions s
    JOIN descendants d ON s.parent_session_id = d.id
)
SELECT id, title, started_at FROM descendants;

Query: Recent Sessions with Preview

SELECT s.*,
    COALESCE(
        (SELECT SUBSTR(m.content, 1, 63)
         FROM messages m
         WHERE m.session_id = s.id AND m.role = 'user' AND m.content IS NOT NULL
         ORDER BY m.timestamp, m.id LIMIT 1),
        ''
    ) AS preview,
    COALESCE(
        (SELECT MAX(m2.timestamp) FROM messages m2 WHERE m2.session_id = s.id),
        s.started_at
    ) AS last_active
FROM sessions s
ORDER BY s.started_at DESC
LIMIT 20;

Query: Token Usage Statistics

-- Total tokens by model
SELECT model,
       COUNT(*) as session_count,
       SUM(input_tokens) as total_input,
       SUM(output_tokens) as total_output,
       SUM(estimated_cost_usd) as total_cost
FROM sessions
WHERE model IS NOT NULL
GROUP BY model
ORDER BY total_cost DESC;

-- Sessions with highest token usage
SELECT id, title, model, input_tokens + output_tokens AS total_tokens,
       estimated_cost_usd
FROM sessions
ORDER BY total_tokens DESC
LIMIT 10;

Export and Cleanup

# Export a single session with messages
data = db.export_session("sess_abc123")

# Export all sessions (with messages) as list of dicts
all_data = db.export_all(source="cli")

# Delete old sessions (only ended sessions)
deleted_count = db.prune_sessions(older_than_days=90)
deleted_count = db.prune_sessions(older_than_days=30, source="telegram")

# Clear messages but keep the session record
db.clear_messages("sess_abc123")

# Delete session and all messages
db.delete_session("sess_abc123")

Database Location

Default path: ~/.hermes/state.db

This is derived from hermes_constants.get_hermes_home() which resolves to ~/.hermes/ by default, or the value of HERMES_HOME environment variable.

The database file, WAL file (state.db-wal), and shared-memory file (state.db-shm) are all created in the same directory.