Open Second Brain

Open Second Brain is a memory layer for AI agents that lives in an Obsidian vault. Agents read and write it through MCP servers, lifecycle hooks and the o2b CLI. Everything the agent learns is a Markdown file under Brain/, so you can open, edit, grep and version it like any other note. A deterministic dream pass turns repeated signals into rules and retires the ones nothing uses; no model runs inside that algorithm.

How it fits

flowchart LR
    subgraph Clients["Agent clients"]
        Plug["Plugins: Claude Code, Codex,<br/>Hermes, opencode, Grok Build"]
        Mcp["MCP config: Cursor, kiro, Copilot CLI,<br/>Gemini CLI, ZCode, any MCP host"]
        Claw["OpenClaw<br/>in-process plugin"]
        Cliu["Aider, Pi<br/>CLI and context file"]
    end
    subgraph Product["Open Second Brain"]
        Srv["MCP servers<br/>open-second-brain, open-second-brain-writer"]
        Hooks["Lifecycle hooks<br/>o2b-hook"]
        Cli["CLI<br/>o2b"]
        Core["Core"]
    end
    subgraph Vault["Your vault"]
        Brain["Brain/<br/>Markdown the agent writes"]
        Notes["Your notes"]
        Index[(".open-second-brain/brain.sqlite<br/>rebuildable search index")]
    end
    Plug --> Srv
    Plug --> Hooks
    Mcp --> Srv
    Claw --> Core
    Cliu --> Cli
    Srv --> Core
    Hooks --> Core
    Cli --> Core
    Core --> Brain
    Core --> Index
    Core -. reads, writes a named note on request .-> Notes

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Each client reaches the same core through a plugin, an MCP registration or the CLI. The core keeps its own records under Brain/ and a generated search index under .open-second-brain/ that it can rebuild from the files. Your notes are read; the note tools write one only at a path the caller names.

flowchart LR
    Agent["Agent"] -->|brain_feedback| Inbox["Brain/inbox/<br/>signal"]
    Inbox -->|dream pass| Pref["Brain/preferences/<br/>rule"]
    Pref -->|rendered into| Active["Brain/active.md"]
    Active -->|read at session start| Agent
    Agent -->|brain_apply_evidence| Log["Brain/log/"]
    Log -->|next dream pass| Pref

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The learning loop: signals become rules, rules reach the next session through Brain/active.md, and applied or violated evidence confirms, quarantines or retires them. Details: docs/how-it-works.md.

Install

1. Install Bun 1.1 or later: install/prerequisites.md.

2. Install the plugin for your client (guides below), or get the o2b CLI: install/prerequisites.md.

3. Create the vault files:

   o2b init       --vault /path/to/vault --name "My Second Brain"
   o2b brain init --vault /path/to/vault

4. Connect the client (table below), then check the result:

   o2b doctor --vault /path/to/vault
   o2b install --check

The full router with readiness criteria is install.md; native Windows is covered in install/windows.md.

Supported clients

Client	Integration	Guide
Claude Code	Marketplace plugin: MCP servers and hooks	install/claudecode.md
OpenAI Codex	Marketplace plugin, then o2b install --target codex --apply	install/codex.md
Hermes Agent	Plugin and memory provider	install/hermes.md
OpenClaw	Native plugin, no MCP	install/openclaw.md
opencode	o2b install --target opencode --apply: MCP servers and a native plugin	install/opencode.md
Grok Build	o2b install --target grok --apply: MCP servers and hooks	install/grok.md
Cursor	o2b install --target cursor --apply	install/cursor.md
kiro	o2b install --target kiro --apply	install/kiro.md
GitHub Copilot CLI	o2b install --target copilot-cli --apply	install/copilot-cli.md
Gemini CLI	o2b install --target gemini-cli --apply	install/gemini-cli.md
ZCode	MCP servers added to its config by hand	install/zcode.md
Aider	o2b install --target aider --apply: context file, no MCP	install/aider.md
Pi	o2b install --target pi --apply: skill over the CLI	install/pi.md
Any other MCP host	o2b install --target generic --apply --out - prints the server entries	install/generic.md

o2b uninstall --target <name> --apply removes what o2b install wrote; the vault is never touched (install.md).

Capabilities

• Preferences that learn and expire. Signals, a deterministic dream pass, confidence and retirement: how it works.
• Reversible changes. Brain mutations take a snapshot first; o2b brain rollback restores it: snapshots.
• Attributed writes. o2b brain writes lists every note write by agent and device, writes revert plans a restore, o2b brain freeze stops all writers, and o2b brain log verify checks the hash-chained log: Brain CLI.
• Search. Keyword search with an optional semantic lane, result explanations, recall profiles and compact cards: search CLI, ranking.
• Session recall. Import agent transcripts already on disk and search them: session logs.
• Hygiene. o2b brain hygiene scan finds contested, duplicate, stale and unused memories; apply runs only the findings you pick: maintenance.
• Claim ledger. o2b brain truth ingest records an entity claim with an optional validity window (--valid-from/--valid-until; with the flags absent, a window on the source record's frontmatter is adopted and frozen at ingest), truth events recalls a windowed slice of the ledger, and truth state grounds an agent-stated claim with an anchoring verdict - committed, or reported back with reason codes. truth state accepts an optional opaque --sample-id naming the context-pack sample the claim drew on, and claim reads report the receipt and outcome provenance behind it at the caller's reach - "receipt known, outcome unknown", never an invented verdict: entity truth CLI.
• Corrections. o2b brain lifecycle correct <target> reports a correction's blast radius and retires the affected records - validity-closed by default, tombstoned with --flatly-wrong, dry run by default and --apply to write. A retired record that recall can still serve is served only beside its correction, never alone: belief lifecycle CLI.
• Sharing and backup. Knowledge packs export a chosen subset; o2b brain bank-export writes a one-file backup bundle: knowledge packs.
• Secret custody. Credentials live encrypted in the vault's state dir behind a capability-gated store; $secret:NAME references resolve at the use sites, o2b brain secret unlock wraps the keyfile under a passphrase, and secret export/import move the store between installs as one passphrase-encrypted bundle: custody CLI.
• Today view and note markers. o2b brain today, @osb loop and @osb set markers: Brain CLI.
• Self-inspection. o2b version, o2b install --friction and o2b state status report what is installed and where state lives: CLI reference.
• MCP surface. Tools, tool profiles for hosts with tool limits, and the always-loaded writer server: docs/mcp.md.

Optional features

• Semantic search: an embedding provider plus sqlite-vec; the embeddings-setup skill walks through it: skills/embeddings-setup/SKILL.md.
• Decision models: a typed judgment model that can rerank search and filter candidates, off by default per use: docs/decision-models.md.
• Deep relational recall: a fourth search arm over typed links, off by default. Its traversal runs under width budgets - 8 seeds, 4 edges per node, 16 nodes in total, and a hub above 12 walked edges is reached but not expanded - each overridable through an OPEN_SECOND_BRAIN_SEARCH_TRAVERSAL_* environment variable or a search_traversal_* config key; entity co-occurrence bridges join the walk by default and switch off separately (OPEN_SECOND_BRAIN_SEARCH_ENTITY_BRIDGES): retrieval quality.
• Staged review for agent writes, off by default. With no key set every write publishes exactly as before. write_approval.notes / OPEN_SECOND_BRAIN_WRITE_APPROVAL_NOTES_ENABLED and write_approval.ingest / OPEN_SECOND_BRAIN_WRITE_APPROVAL_INGEST_ENABLED (each falling back to the write_approval.enabled master, default off) stage note creates and ingest summary pages into Brain/pending/ beside the staged signals, where they stay out of the search index until an operator runs o2b brain pending list and applies or rejects them; write-path integrity.
• A permissions document, absent by default. Brain/_permissions.yaml (an operator-edited vault file) resolves allow/ask/deny per agent, role and target for the write, ingest and owner-write actions; with the file absent every check behaves exactly as today. ask stages the write, deny refuses with the write-refused token and the next command o2b brain permissions show, and every ask/deny verdict lands in a queryable decision ledger under Brain/logs/decisions/; o2b brain permissions show dry-runs the decision table, ledger reads the rows; Brain CLI.
• The owner-write gate, off by default. integrity.owner_scope_writes in Brain/_brain.yaml (off | warn | fail, default off) refuses a caller-named owner that differs from the caller's resolved identity on the preference and note lanes (warn allows and records one decision-ledger row); a document owner_write verdict composes most-restrictive-wins with the gate; write-time integrity.
• Named MCP tokens, optional. o2b mcp token mint|rotate|revoke|list keeps a hash-at-rest token per agent (.open-second-brain/secrets/mcp-tokens.json; material shown exactly once). Over HTTP a valid token authenticates as its agent per request, the shared --api-key stays valid as the operator master credential, and mcp_tokens_required / OPEN_SECOND_BRAIN_MCP_TOKENS_REQUIRED (default false) makes a non-empty token map refuse credential-less requests. o2b bootstrap --target <harness> [--token] [--rotate] [--check] provisions MCP registration, token and receipt in one idempotent command, and o2b bootstrap --remove <harness> tears a provision down again; the minted token authenticates HTTP clients configured by hand, while the registered stdio harness presents no credential and keeps its config-derived identity; core CLI.
• Ambient capture consent, opt-out. guardrails.ambient_writeback: false suppresses the ambient extraction lane with a counted ambient-withheld event (absent keeps today's behavior), and guardrails.ambient_ttl_days stamps an expiration_date on ambient-extracted signals so reads drop them after the window (absent stamps nothing); a TTL-stamped signal still stages when the review gate is on and survives apply verbatim.
• Open decisions. o2b brain decision open --title <t> --question <q> --option <o> [...] parks a question with enumerated options at Brain/decisions/open-<slug>.md; resolve mints the real type: decision page, discard closes without deciding, and the morning brief renders up to five open questions: belief lifecycle.

What is new

1.81.0 is the answer-trust release: it narrows the gap between what an agent was given and what its answer can be believed on. A Stop-hook step verifies the quotations in the assistant's answer against the note cited for them or the evidence recall actually injected (quote_check.recall|active|subagent in Brain/_brain.yaml, each off|shadow|enforce, default shadow - annotate-only, one additionalContext note per turn; enforce adds one Brain-log record per turn). o2b brain truth state accepts an optional opaque --sample-id naming the context-pack sample a claim drew on, and claim reads report the receipt and outcome provenance behind it at the caller's reach. Trusted local-file ingest now archives a source's bytes content-addressed under Brain/.archives/ (redacted before persist, refused past 1 MiB, never pruned) with the hash stamped on the summary page; the capture-scope hygiene detector and a count-only doctor section name missing or mismatched evidence, and brain_hygiene archive_get reads an entry back at local reach. A record with no session id is read as no-session everywhere - scoped recall excludes it, grouping skips it, the session digest omits the key - dissolving the phantom blank-session bucket, with a count-only blank-session-identity doctor check for what older writers left behind. A repair lane closes the outstanding review findings: attribute narrowing in search falls a document whose indexed snapshot went stale through to the live filter instead of silently excluding it. Every release is described in the CHANGELOG.

Documentation

Topic	Page
Mental model, vault layout, dream pass, safety properties	docs/how-it-works.md
Every CLI verb and flag	docs/cli-reference.md
MCP protocol, tools, writer split	docs/mcp.md
Architecture and configuration model	docs/architecture.md
Updating (o2b update) and upgrade notes	docs/updating.md
Observability events	docs/observability.md
Metrics data contract	docs/metrics.md
Cross-project pointer	docs/cross-project-pointer.md
Scheduled Brain digest (Hermes cron)	docs/hermes-cron.md
Stability policy	docs/stability.md
Origin of the idea	docs/idea.md

Project

CHANGELOG · Security policy · Source on GitHub · MIT License