hermes-agent/website/docs/developer-guide/context-compression-and-caching.md

Context Compression and Caching

Hermes Agent uses a dual compression system and Anthropic prompt caching to manage context window usage efficiently across long conversations.

Source files: agent/context_engine.py (ABC), agent/context_compressor.py (default engine), agent/prompt_caching.py, gateway/run.py (session hygiene), run_agent.py (search for _compress_context)

Pluggable Context Engine

Context management is built on the ContextEngine ABC (agent/context_engine.py). The built-in ContextCompressor is the default implementation, but plugins can replace it with alternative engines (e.g., Lossless Context Management).

context:
  engine: "compressor"    # default — built-in lossy summarization
  engine: "lcm"           # example — plugin providing lossless context

The engine is responsible for:

• Deciding when compaction should fire (should_compress())
• Performing compaction (compress())
• Optionally exposing tools the agent can call (e.g., lcm_grep)
• Tracking token usage from API responses

Selection is config-driven via context.engine in config.yaml. The resolution order:

1. Check plugins/context_engine/<name>/ directory
2. Check general plugin system (register_context_engine())
3. Fall back to built-in ContextCompressor

Plugin engines are never auto-activated — the user must explicitly set context.engine to the plugin's name. The default "compressor" always uses the built-in.

Configure via hermes plugins → Provider Plugins → Context Engine, or edit config.yaml directly.

For building a context engine plugin, see Context Engine Plugins.

Dual Compression System

Hermes has two separate compression layers that operate independently:

                     ┌──────────────────────────┐
  Incoming message   │   Gateway Session Hygiene │  Fires at 85% of context
  ─────────────────► │   (pre-agent, rough est.) │  Safety net for large sessions
                     └─────────────┬────────────┘
                                   │
                                   ▼
                     ┌──────────────────────────┐
                     │   Agent ContextCompressor │  Fires at 50% of context (default)
                     │   (in-loop, real tokens)  │  Normal context management
                     └──────────────────────────┘

1. Gateway Session Hygiene (85% threshold)

Located in gateway/run.py (search for Session hygiene: auto-compress). This is a safety net that runs before the agent processes a message. It prevents API failures when sessions grow too large between turns (e.g., overnight accumulation in Telegram/Discord).

• Threshold: Fixed at 85% of model context length
• Token source: Prefers actual API-reported tokens from last turn; falls back to rough character-based estimate (estimate_messages_tokens_rough)
• Fires: Only when len(history) >= 4 and compression is enabled
• Purpose: Catch sessions that escaped the agent's own compressor

The gateway hygiene threshold is intentionally higher than the agent's compressor. Setting it at 50% (same as the agent) caused premature compression on every turn in long gateway sessions.

2. Agent ContextCompressor (50% threshold, configurable)

Located in agent/context_compressor.py. This is the primary compression system that runs inside the agent's tool loop with access to accurate, API-reported token counts.

Configuration

All compression settings are read from config.yaml under the compression key:

compression:
  enabled: true              # Enable/disable compression (default: true)
  threshold: 0.50            # Fraction of context window (default: 0.50 = 50%)
  # model_thresholds:        # Per-model threshold overrides (substring match,
  #   "glm-5.2": 0.40        # longest key wins). See "Per-model threshold
  #   "claude-sonnet": 0.35  # overrides" below.
  target_ratio: 0.20         # How much of threshold to keep as tail (default: 0.20)
  protect_last_n: 20         # Minimum protected tail messages (default: 20)
  codex_gpt55_autoraise: true  # gpt-5.5 on Codex OAuth: raise trigger to 85% (default: true)
  codex_gpt55_autoraise_notice: true  # Show the one-time autoraise notice (default: true)
  codex_app_server_auto: native  # native|hermes|off for Codex app-server thread compaction

# Summarization model/provider configured under auxiliary:
auxiliary:
  compression:
    model: null              # Override model for summaries (default: auto-detect)
    provider: auto           # Provider: "auto", "openrouter", "nous", "main", etc.
    base_url: null           # Custom OpenAI-compatible endpoint

Parameter Details

Parameter	Default	Range	Description
threshold	0.50	0.0-1.0	Compression triggers when prompt tokens ≥ threshold × context_length
model_thresholds	{}	map	Per-model overrides of threshold. Keys are substring-matched against the model name (longest match wins). The small-context floor still applies on top (see below)
target_ratio	0.20	0.10-0.80	Controls tail protection token budget: threshold_tokens × target_ratio
protect_last_n	20	≥1	Minimum number of recent messages always preserved
protect_first_n	3	(hardcoded)	System prompt + first exchange always preserved
codex_gpt55_autoraise	true	bool	Raise the trigger to 85% for gpt-5.5 on the ChatGPT Codex OAuth route (see below). Set false to keep the global threshold
codex_gpt55_autoraise_notice	true	bool	Show the one-time Codex gpt-5.5 autoraise notice. Set false to keep the 85% autoraise but suppress the banner
codex_app_server_auto	native	native, hermes, off	Thread-compaction mode for Codex app-server sessions (see below)

Per-model threshold overrides

compression.model_thresholds lets you trigger compaction at different points depending on the active model — useful when you swap between models with very different context windows (e.g. a 1M-context model can compress later while a 128K model should compress earlier):

compression:
  threshold: 0.50
  model_thresholds:
    "glm-5.2": 0.40
    "glm-5.2-1M": 0.25
    "claude-sonnet": 0.35

Resolution rules:

• Keys are substring-matched against the model name; the longest matching key wins (glm-5.2-1M beats glm-5.2 for model glm-5.2-1M).
• When no key matches (or the map is empty), the global threshold applies.
• The override is re-resolved on every /model switch; switching to a model with no matching key falls back to the global threshold.
• The small-context floor still applies on top of overrides (raise-only): models with context windows below 512K are floored at 0.75, so an override below the floor is raised to 0.75, while an override above it (e.g. 0.80) wins.

Plugin context engines can reuse the same resolution logic via from agent.context_compressor import resolve_model_threshold; engines that override update_model() own their own compaction policy and may ignore the map.

Codex gpt-5.5 threshold autoraise

The ChatGPT Codex OAuth backend hard-caps gpt-5.5 at a 272K context window (the same slug exposes 1.05M on OpenAI's direct API and OpenRouter, and 400K on GitHub Copilot). At the default 50% trigger, compaction would fire at ~136K — half the window the model can actually use. When the active route is Codex OAuth (provider: openai-codex) and the model is gpt-5.5, Hermes raises the trigger to 85% (~231K) and shows a notice with the opt-out command. The notice is shown once per profile — a marker under $HERMES_HOME (.codex_gpt55_autoraise_notice) records that it ran, so repeated agent/session inits (e.g. every inbound gateway message) don't re-emit it; if the raised threshold later changes it re-notifies once. Only this exact route is affected; gpt-5.5 on any other provider keeps your global threshold. To opt back down to the global value:

hermes config set compression.codex_gpt55_autoraise false

To keep the 85% autoraise but hide only the one-time notice:

hermes config set compression.codex_gpt55_autoraise_notice false

Codex app-server thread compaction

Codex app-server sessions (api_mode: codex_app_server — the codex CLI/agent runtime) are different from every other route: the codex agent owns the backing thread context, so Hermes' auxiliary summarizer cannot shrink it — rewriting the local transcript mirror leaves the real thread growing unbounded until a hard context reset. For this runtime, compaction goes through the app-server's own mechanism instead:

• Manual compaction (/compress) asks the app-server to compact the thread (thread/compact/start) and waits for the compaction turn to complete.
• Automatic compaction is controlled by compression.codex_app_server_auto: the default native lets the app-server decide when to compact and Hermes records the resulting compaction events (compression counters, session events). Set hermes to let Hermes' compression threshold initiate app-server compaction, or off to disable Hermes-initiated automatic compaction entirely (codex may still compact natively).

Hermes' local transcript is never rewritten on this runtime — state.db records the compaction boundary while the visible transcript stays intact. All other routes (including Codex OAuth chat sessions) keep Hermes' summary compressor.

Computed Values (for a 200K context model at defaults)

context_length       = 200,000
threshold_tokens     = 200,000 × 0.50 = 100,000
tail_token_budget    = 100,000 × 0.20 = 20,000
max_summary_tokens   = min(200,000 × 0.05, 12,000) = 10,000

:::note Threshold is derived from the MAIN model's context window threshold_tokens is always threshold × context_length, where context_length is the main agent model's context window — never the auxiliary/summary model's. On a 262,144-token model at the default 0.50, the threshold is 262,144 × 0.50 = 131,072. That number being close to a common "128K context" is a coincidence of the percentage, not a sign that the auxiliary model's window is the trigger. The auxiliary model's context window is a separate concern — see the "Summary model context length" warning below for how it affects whether a summary can be produced, not when compression fires. :::

Compression Algorithm

The ContextCompressor.compress() method follows a 4-phase algorithm:

Phase 1: Prune Old Tool Results (cheap, no LLM call)

Old tool results (>200 chars) outside the protected tail are replaced with:

[Old tool output cleared to save context space]

This is a cheap pre-pass that saves significant tokens from verbose tool outputs (file contents, terminal output, search results).

Phase 2: Determine Boundaries

┌─────────────────────────────────────────────────────────────┐
│  Message list                                               │
│                                                             │
│  [0..2]  ← protect_first_n (system + first exchange)        │
│  [3..N]  ← middle turns → SUMMARIZED                        │
│  [N..end] ← tail (by token budget OR protect_last_n)        │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Tail protection is token-budget based: walks backward from the end, accumulating tokens until the budget is exhausted. Falls back to the fixed protect_last_n count if the budget would protect fewer messages.

Boundaries are aligned to avoid splitting tool_call/tool_result groups. The _align_boundary_backward() method walks past consecutive tool results to find the parent assistant message, keeping groups intact.

Phase 3: Generate Structured Summary

:::warning Summary model context length The summary model must have a context window at least as large as the main agent model's. The entire middle section is sent to the summary model in a single call_llm(task="compression") call. If the summary model's context is smaller, the API returns a context-length error — _generate_summary() catches it, logs a warning, and returns None. The compressor then drops the middle turns without a summary, silently losing conversation context. This is the most common cause of degraded compaction quality. :::

The middle turns are summarized using the auxiliary LLM with a structured template:

## Goal
[What the user is trying to accomplish]

## Constraints & Preferences
[User preferences, coding style, constraints, important decisions]

## Progress
### Done
[Completed work — specific file paths, commands run, results]
### In Progress
[Work currently underway]
### Blocked
[Any blockers or issues encountered]

## Key Decisions
[Important technical decisions and why]

## Relevant Files
[Files read, modified, or created — with brief note on each]

## Next Steps
[What needs to happen next]

## Critical Context
[Specific values, error messages, configuration details]

Summary budget scales with the amount of content being compressed:

• Formula: content_tokens × 0.20 (the _SUMMARY_RATIO constant)
• Minimum: 2,000 tokens
• Maximum: min(context_length × 0.05, 12,000) tokens

Phase 4: Assemble Compressed Messages

The compressed message list is:

1. Head messages (with a note appended to system prompt on first compression)
2. Summary message (role chosen to avoid consecutive same-role violations)
3. Tail messages (unmodified)

Orphaned tool_call/tool_result pairs are cleaned up by _sanitize_tool_pairs():

• Tool results referencing removed calls → removed
• Tool calls whose results were removed → stub result injected

Iterative Re-compression

On subsequent compressions, the previous summary is passed to the LLM with instructions to update it rather than summarize from scratch. This preserves information across multiple compactions — items move from "In Progress" to "Done", new progress is added, and obsolete information is removed.

The _previous_summary field on the compressor instance stores the last summary text for this purpose.

Before/After Example

Before Compression (45 messages, ~95K tokens)

[0] system:    "You are a helpful assistant..." (system prompt)
[1] user:      "Help me set up a FastAPI project"
[2] assistant: <tool_call> terminal: mkdir project </tool_call>
[3] tool:      "directory created"
[4] assistant: <tool_call> write_file: main.py </tool_call>
[5] tool:      "file written (2.3KB)"
    ... 30 more turns of file editing, testing, debugging ...
[38] assistant: <tool_call> terminal: pytest </tool_call>
[39] tool:      "8 passed, 2 failed\n..."  (5KB output)
[40] user:      "Fix the failing tests"
[41] assistant: <tool_call> read_file: tests/test_api.py </tool_call>
[42] tool:      "import pytest\n..."  (3KB)
[43] assistant: "I see the issue with the test fixtures..."
[44] user:      "Great, also add error handling"

After Compression (25 messages, ~45K tokens)

[0] system:    "You are a helpful assistant...
               [Note: Some earlier conversation turns have been compacted...]"
[1] user:      "Help me set up a FastAPI project"
[2] assistant: "[CONTEXT COMPACTION] Earlier turns were compacted...

               ## Goal
               Set up a FastAPI project with tests and error handling

               ## Progress
               ### Done
               - Created project structure: main.py, tests/, requirements.txt
               - Implemented 5 API endpoints in main.py
               - Wrote 10 test cases in tests/test_api.py
               - 8/10 tests passing

               ### In Progress
               - Fixing 2 failing tests (test_create_user, test_delete_user)

               ## Relevant Files
               - main.py — FastAPI app with 5 endpoints
               - tests/test_api.py — 10 test cases
               - requirements.txt — fastapi, pytest, httpx

               ## Next Steps
               - Fix failing test fixtures
               - Add error handling"
[3] user:      "Fix the failing tests"
[4] assistant: <tool_call> read_file: tests/test_api.py </tool_call>
[5] tool:      "import pytest\n..."
[6] assistant: "I see the issue with the test fixtures..."
[7] user:      "Great, also add error handling"

Prompt Caching (Anthropic)

Source: agent/prompt_caching.py

Reduces input token costs by ~75% on multi-turn conversations by caching the conversation prefix. Uses Anthropic's cache_control breakpoints.

Strategy: system_and_3

Anthropic allows a maximum of 4 cache_control breakpoints per request. Hermes uses the "system_and_3" strategy:

Breakpoint 1: System prompt           (stable across all turns)
Breakpoint 2: 3rd-to-last non-system message  ─┐
Breakpoint 3: 2nd-to-last non-system message   ├─ Rolling window
Breakpoint 4: Last non-system message          ─┘

How It Works

apply_anthropic_cache_control() deep-copies the messages and injects cache_control markers:

# Cache marker format
marker = {"type": "ephemeral"}
# Or for 1-hour TTL:
marker = {"type": "ephemeral", "ttl": "1h"}

The marker is applied differently based on content type:

Content Type	Where Marker Goes
String content	Converted to [{"type": "text", "text": ..., "cache_control": ...}]
List content	Added to the last element's dict
None/empty	Added as msg["cache_control"]
Tool messages	Added as msg["cache_control"] (native Anthropic only)

Cache-Aware Design Patterns

1. Stable system prompt: The system prompt is breakpoint 1 and cached across all turns. Avoid mutating it mid-conversation (compression appends a note only on the first compaction).

2. Message ordering matters: Cache hits require prefix matching. Adding or removing messages in the middle invalidates the cache for everything after.

3. Compression cache interaction: After compression, the cache is invalidated for the compressed region but the system prompt cache survives. The rolling 3-message window re-establishes caching within 1-2 turns.

4. TTL selection: Default is 5m (5 minutes). Use 1h for long-running sessions where the user takes breaks between turns.

5. Model identity is part of the cache key: Provider-side caches are scoped to the model (and account/API key) serving the request. Any mid-conversation model change — an explicit /model switch, primary-model fallback, or a credential-pool rotation onto a different account — means the next request gets zero cache hits and re-reads the full conversation at undiscounted input price. This is inherent to how provider caches work, not something Hermes can avoid; user-facing docs for /model, fallback providers, and credential pools carry cost warnings for this reason. Don't add features that silently swap the model or credentials mid-session.

Enabling Prompt Caching

Prompt caching is automatically enabled when:

• The model is an Anthropic Claude model (detected by model name)
• The provider supports cache_control (native Anthropic API or OpenRouter)

# config.yaml — TTL is configurable (must be "5m" or "1h")
prompt_caching:
  cache_ttl: "5m"

The CLI shows caching status at startup:

💾 Prompt caching: ENABLED (Claude via OpenRouter, 5m TTL)

Context Pressure Warnings

Intermediate context-pressure warnings have been removed (see the iteration-budget block in run_agent.py, which notes: "No intermediate pressure warnings — they caused models to 'give up' prematurely on complex tasks"). Compression fires when prompt tokens reach the configured compression.threshold (default 50%) with no prior warning step; gateway session hygiene fires as the secondary safety net at 85% of the model's context window.