history-graph-protocol 0.3.3

An MCP server that gives AI agents a permanent, append-only causal history

History Graph Protocol (HGP)

An MCP server that gives AI agents a permanent, append-only causal history.

HGP records what an agent did and why — every file write, every decision, every piece of evidence behind each action. Where memory systems store what an agent knows, HGP stores what an agent did. Think of other memory systems as working memory; HGP is the audit trail.

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Why HGP?

Without HGP	With HGP
Agent writes a file — no record of why	Every file write is a traceable artifact
Decision rationale lives only in context	Hypotheses and evidence are permanently stored
Can't tell what changed or when	Full causal graph: who did what, citing what
Session ends, history is gone	History persists across sessions and agents

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Core Concepts

Operations (Nodes)

Every action is an immutable, append-only record. Four types:

Type	When to use
artifact	A produced output — file written, data generated
hypothesis	A decision, plan, or inference
merge	Combining multiple prior operations
invalidation	Superseding or retracting a prior operation

Operations are never deleted or mutated. The graph only grows.

Causal Graph (DAG)

Operations are connected by directed edges:

• causal — A produced B (B depends on A)
• invalidates — A supersedes B

graph LR
    A["op-1\nartifact\n(analysis)"]
    B["op-2\nhypothesis\n(decision)"]
    C["op-3\ninvalidation\n(revised decision)"]

    A -->|causal| B
    C -->|invalidates| B
    A -->|causal| C

Memory Tier

Each operation has a memory tier that reflects access recency:

Tier	Meaning
short_term	Recently active; included in all queries
long_term	Older but still reachable; included in all queries
inactive	Not accessed recently; excluded from default queries, never deleted

Tiers update automatically and can be set manually via hgp_set_memory_tier.

Evidence Trail

Operations can cite other operations as evidence, independently of the DAG edge structure. Evidence relations carry:

• relation — supports, refutes, context, method, or source
• scope — which aspect of the citing op this evidence applies to
• inference — free-text explanation of how the evidence was used

Given any decision, you can reconstruct exactly what evidence it was based on and how each piece was interpreted.

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Installation

Prerequisites: Python ≥ 3.12, uv

pip install history-graph-protocol

The PyPI package is history-graph-protocol; the installed CLI command is hgp.

One-command setup

hgp install

This registers HGP as an MCP server, installs hooks, and injects agent instructions — all in one step. Supports Claude Code, Gemini CLI, and Codex:

hgp install            # all supported clients (recommended)
hgp install --claude   # Claude Code only
hgp install --gemini   # Gemini CLI only
hgp install --codex    # Codex only
hgp install --local    # project-local scope instead of global

After running, restart your client and verify HGP tools are available.

Manual setup

Configure without hgp install

Claude Code:

claude mcp add --scope user hgp -- python -m hgp.server

Gemini CLI:

gemini mcp add --scope user hgp python -m hgp.server

Codex — add to .codex/config.toml:

[mcp_servers.hgp]
command = "python"
args = ["-m", "hgp.server"]

Use the Python that has history-graph-protocol installed. If using a virtual environment, replace python with the absolute path to the venv Python (e.g. /path/to/.venv/bin/python).

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Quick Start

These are MCP tool calls — invoke them through your MCP client (Claude Code, Gemini CLI, or any MCP-compatible host).

# Write a file and record it as an artifact
hgp_write_file(
    file_path="src/analysis.py",
    content="...",
    agent_id="claude-code",
    metadata={"description": "Initial analysis script"},
)
# → returns op_id: "op-abc123"

# Record the decision that led to a follow-up file
decision = hgp_create_operation(
    op_type="hypothesis",
    agent_id="claude-code",
    metadata={"description": "Refactor to use async based on profiling results"},
    parent_op_ids=["op-abc123"],
)

# Write the refactored file, linked to the decision
hgp_write_file(
    file_path="src/analysis.py",
    content="...",
    agent_id="claude-code",
    parent_op_ids=[decision["op_id"]],
)

# Later: audit what happened to a file
hgp_file_history(file_path="src/analysis.py")
# → all HGP operations recorded for this file, in order

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Tool Index

File tools

Tool	Description
hgp_write_file	Write (create or overwrite) a file and record it as an artifact
hgp_append_file	Append content to a file and record as artifact
hgp_edit_file	Replace a unique string in a file and record as artifact
hgp_delete_file	Delete a file and record an invalidation operation
hgp_move_file	Move/rename a file; records invalidation of old path + new artifact
hgp_file_history	Return all HGP operations recorded for a given file path

Graph tools

Tool	Description
hgp_create_operation	Record a new operation; optionally attach payload, link parents, cite evidence
hgp_query_operations	Filter operations by type, agent, status, or memory tier
hgp_query_subgraph	Traverse ancestors or descendants from a root operation
hgp_get_evidence	List all operations a given op cited as evidence
hgp_get_citing_ops	Reverse lookup — list all ops that cited a given op as evidence
hgp_get_artifact	Retrieve binary payload from CAS by its object_hash
hgp_anchor_git	Link an operation to a Git commit SHA (requires full 40-char lowercase hex SHA)
hgp_set_memory_tier	Manually set an operation's memory tier (short_term, long_term, inactive)

Lease tools

Leases provide optimistic locking for multi-step write sequences.

Tool	Description
hgp_acquire_lease	Acquire a lock on a subgraph before multi-step writes
hgp_validate_lease	Check a lease is still active; extends TTL by default
hgp_release_lease	Release a lease explicitly after writing

Maintenance

Tool	Description
hgp_reconcile	Run crash-recovery reconciler (use after unexpected shutdown)

Token-sensitive sessions: mutation tools accept verbose=False to omit chain_hash and object_hash from responses, reducing per-call token overhead by ~73%. Default is verbose=True.

→ Full API reference: docs/tools-reference.md
→ Usage patterns and examples: docs/usage-patterns.md

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Configuration

HGP mode

Control whether HGP records operations. Mode is stored in <repo_root>/.hgp/mode. Agents are blocked from changing it via the hgp mode Bash command — the pre-Bash hook intercepts and rejects those calls.

hgp mode              # show current mode (default: on)
hgp mode on           # normal operation
hgp mode advisory     # mutation tools return HGP_ADVISORY instead of recording
hgp mode off          # all tools return HGP_DISABLED

Mode	Mutation tools	Query tools
on	execute normally	execute normally
advisory	return {"status": "HGP_ADVISORY"}	execute normally
off	return {"status": "HGP_DISABLED"}	return {"status": "HGP_DISABLED"}

Hook enforcement policy

Hooks warn (or block) when native file tools are used instead of HGP equivalents.

hgp hook-policy              # show current policy (default: advisory)
hgp hook-policy advisory     # warn only — native file tools still allowed
hgp hook-policy block        # block native Write/Edit/MultiEdit/write_file/replace

If you installed hooks before this feature was added, run hgp install again to update them.

Storage

HGP stores its database and content-addressable blobs in <repo_root>/.hgp/ (gitignored). The server resolves the project root from the nearest .git directory at startup.

• Graph tools (hgp_create_operation, queries, etc.): if no git repo is found, fall back to <cwd>/.hgp/ with a warning.
• File tools (hgp_write_file, hgp_edit_file, etc.): always require a git repo or an explicit HGP_PROJECT_ROOT. Without one, they return PROJECT_ROOT_NOT_FOUND.

The .hgp/ directory is reserved for HGP internals. File tools reject any path that contains a .hgp directory segment — including nested paths like sub/.hgp/file.txt — and return HGP_INTERNAL_PATH. This applies across all .hgp/ directories in a project tree, which is intentional for monorepo and nested-repo layouts.

One server process is bound to one store.

Environment variables

Variable	Default	Description
HGP_PROJECT_ROOT	(auto)	Override project root (default: nearest .git from cwd, then cwd)
HGP_GLOBAL_MODE	(unset)	Set to 1 to force global store at ~/.hgp/ (all projects share one DB)
HGP_HOOK_BLOCK	0	Set to 1 to block native file tool calls instead of warning

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Run as MCP server

hgp
# or
python -m hgp.server

HGP uses stdio transport and is compatible with any MCP-compliant host.