Context Firewall

Why it exists

Large tool ecosystems produce large, verbose outputs. Passing raw tool output to an LLM causes context blowup, leaks PII, and makes the agent unpredictable. The firewall transforms every RawResult into a bounded Frame before the LLM sees it.

Budgets

from weaver_kernel.firewall.budgets import Budgets

Budgets(
    max_rows=50,    # max rows in table_preview
    max_fields=20,  # max fields per row
    max_chars=4000, # total characters across all facts
    max_depth=3,    # recursion depth for nested structures
)

The character size used for budget comparisons is computed by an allocation-free estimator (weaver_kernel.firewall.estimated_size) that walks the structure rather than serialising it with json.dumps — so a multi-MB raw result is never fully serialised just to measure it. The estimate is deterministic and tracks the serialised length closely; only threshold comparisons depend on it.

Response modes

Mode	What you get	When to use
summary	≤20 fact strings + handle	Default; best for LLM context
table	≤max_rows dicts + handle	When the LLM needs tabular data
handle_only	handle + warnings	Defer all data to an expand() call
raw	Full data (admin only)	Debugging; never for LLM context

Handles

A Handle is an opaque reference to the full dataset stored server-side.

A handle is bound to the principal it was granted to, so expand() requires that same principal — an omitted or mismatched principal raises HandleConstraintViolation (handle IDs are not bearer credentials). See docs/security.md#handle-expansion-boundary.

# Stored automatically on every invoke()
handle = frame.handle

# Expand with pagination
expanded = kernel.expand(handle, query={"offset": 10, "limit": 5}, principal=principal)

# Field selection
expanded = kernel.expand(handle, query={"fields": ["id", "name"]}, principal=principal)

# Basic filtering
expanded = kernel.expand(handle, query={"filter": {"status": "unpaid"}}, principal=principal)

Bounding handle memory by size

The store holds raw, pre-firewall datasets, and entry count is a poor proxy for memory — one deployment's 10k entries are kilobytes, another's are gigabytes. HandleStore accepts two optional byte budgets (both None = disabled, so default behaviour is unchanged):

from weaver_kernel import HandleStore

store = HandleStore(
    max_total_bytes=512 * 1024 * 1024,  # evict oldest-first until within budget
    max_entry_bytes=64 * 1024 * 1024,   # reject a single over-cap payload
)

Sizes are estimated with the same estimated_size walk used for budgets. max_total_bytes evicts oldest-first after each store (never the just-stored entry); max_entry_bytes rejects an over-cap payload with HandleTooLarge rather than truncating it, keeping expansion faithful to the original dataset. A single entry larger than max_total_bytes can never fit, so it is rejected the same way — current_bytes therefore never exceeds max_total_bytes. Expanding an evicted handle raises the usual HandleNotFound. Tighter budgets mean more "handle expired/evicted" experiences — tune for your workload.

Redaction

When a capability has SensitivityTag.PII or SensitivityTag.PCI:

• Fields in Capability.allowed_fields are kept (others removed)
• Sensitive field names (email, phone, card_number, ssn, etc.) are replaced with [REDACTED]
• Inline patterns in string values (email addresses, phone numbers, SSNs, card numbers) are redacted

Principals with the pii_reader role bypass allowed_fields enforcement.

Redaction is applied on every path that returns data to the LLM, not just the first transform():

• Depth boundary (fail-closed). The max_depth cap bounds recursion cost. At the boundary, scalar strings are still pattern-scrubbed, but a nested container is elided ([REDACTED: nested data beyond depth limit]) rather than returned verbatim — a deeply nested subtree never reaches the LLM unscanned.
• Handle expansion. HandleStore.expand() runs its projected rows through the same redact() as the first invocation, so a secret inline in a permitted field (e.g. a token in a note value) is scrubbed on expand too.
• Streaming. Firewall.apply_stream() keeps a per-field StreamRedactor that holds back a trailing overlap window, so a secret split across two chunks is reassembled and redacted before either half is emitted. Patterns containing internal whitespace (phone/SSN/spaced card numbers) split exactly at the held boundary may still evade detection — see docs/security.md.

Invocation arguments recorded on ActionTrace.args, and driver error text, are run through the same redactor before persistence, so the trace store never becomes a sensitive-data sink (see docs/security.md).

Summarization

Summaries are produced deterministically:

• list of dicts → row count + top keys + numeric stats + categorical/boolean distributions
• dict → key list + per-value type/value
• string → truncated to 500 chars
• other → repr() truncated to 200 chars

Boolean columns are reported as True/False counts, never averaged (a bool is an int subclass in Python, so "mean of is_active = 0.7" is nonsense). When the fact list is capped by max_facts, the final fact is an explicit omission marker (… (N more facts omitted; full data via handle)) so a truncated summary is never mistaken for a complete one.

Cross-invocation budgets

The per-invocation Budgets above cap a single Frame. A separate BudgetManager tracks cumulative token usage across invocations within a session. It is optional — if you don't attach one, kernel behavior is unchanged.

from weaver_kernel import BudgetManager, Kernel

manager = BudgetManager(total_budget=100_000)
kernel = Kernel(registry, budget_manager=manager)

Per invoke() the kernel:

1. Reserves a slice of the remaining budget (default 4,000 tokens). If the budget is empty, BudgetExhausted is raised before the driver runs.
2. Consults manager.suggested_mode(requested) to escalate the requested response_mode to a more aggressive tier as the remaining budget shrinks.
3. After the firewall produces a Frame, counts the actual tokens in the LLM-facing payload and reconciles them against the reservation.

Escalation table:

Budget remaining	Suggested mode (effective response_mode)
> 50%	Caller's requested mode (no change)
20% – 50%	table (when caller requested raw)
5% – 20% (≥ 5%)	summary (floor — never relaxes to table)
< 5%	handle_only

Boundaries land in the more-conservative tier — exactly 50% remaining downgrades raw to table, exactly 20% floors at summary, and only when remaining drops below 5% does handle_only take over.

Kernel.invoke(..., dry_run=True) mirrors the escalation and reports budget_remaining in the returned DryRunResult, so callers can preview what their next live invocation would actually return.

The default counter (default_token_counter) is a character-based len(json.dumps(value)) // 4 approximation with no extra dependencies. For real token counts, install the tiktoken extra and use the shipped factory:

from weaver_kernel.firewall import BudgetManager, make_tiktoken_counter

# pip install weaver-kernel[tiktoken]
manager = BudgetManager(
    total_budget=128_000,
    token_counter=make_tiktoken_counter(),              # default cl100k_base
    # token_counter=make_tiktoken_counter("o200k_base"),  # GPT-4o / o-series
)

make_tiktoken_counter resolves and caches the encoder eagerly, so a missing extra (ImportError) or an unknown encoding name (FirewallError) fails at construction rather than mid-budgeting. The encoding is explicit because models tokenize differently — name the one you budget against. tiktoken is imported lazily, so import weaver_kernel never pulls the heavyweight dependency. Any callable matching the TokenCounter protocol works too.

Streaming

For large results that arrive incrementally (e.g. SSE-style HTTP responses, chunked database cursors, line-by-line tool output), Firewall.apply_stream() lets you process chunks one at a time. PII redaction and per-chunk budget caps apply on every yielded Frame — secrets cannot leak just because they arrived in chunk N rather than the final aggregate.

from weaver_kernel.drivers.base import ExecutionContext, StreamingDriver

class MyStreamingDriver:
    driver_id = "stream"

    async def execute(self, ctx: ExecutionContext):
        # one-shot fallback, called when StreamingDriver isn't used.
        ...

    async def execute_stream(self, ctx: ExecutionContext):
        async for row in some_async_cursor(ctx):
            yield {"row": row}
        yield {"__is_final__": True}  # explicit sentinel (optional)


# isinstance(driver, StreamingDriver) is runtime-checkable.
assert isinstance(MyStreamingDriver(), StreamingDriver)

async for frame in kernel.invoke_stream(token, principal=p, args={}):
    handle_chunk(frame)
    if frame.is_final:
        break

When the resolved driver does not implement StreamingDriver, Kernel.invoke_stream falls back to a single Driver.execute() call and yields exactly one Frame with is_final=True. Each invocation produces one ActionTrace covering the whole stream.

Observability

weaver_kernel.instrument_kernel(kernel) installs OpenTelemetry spans and metric emission on Kernel.invoke and Kernel.grant_capability:

from weaver_kernel import Kernel, instrument_kernel, OTEL_AVAILABLE

kernel = Kernel(registry=...)
if OTEL_AVAILABLE:
    instrument_kernel(kernel)  # no-op when [otel] extra not installed

Spans: weaver_kernel.invoke, weaver_kernel.grant. Metrics: weaver_kernel.invocations (counter), weaver_kernel.invocation_duration (histogram, ms), weaver_kernel.policy_denials (counter). The call is idempotent — repeat invocations on the same kernel are no-ops.