intercept¶

Act on intercepted requests, responses, or streaming frames currently held by the proxy. Held items are released, modified and forwarded, or dropped.

The intercept rule engine dispatches via type-switch on the held envelope's Message, routing to per-protocol rule engines (internal/rules/{http,ws,grpc,sse,raw,common}/). The MCP tool surface accepts a discriminated union of typed modify payloads -- exactly one of http, ws, grpc_start, grpc_data, or raw must be supplied for modify_and_forward, and it must match the held envelope's Message type.

Parameters¶

Parameter	Type	Required	Description
`action`	string	Yes	Action to perform: `release`, `modify_and_forward`, `drop`
`params`	object	Yes	Common parameters (see below)
`http`	object	Conditional	Typed modify payload for an `HTTPMessage` envelope
`ws`	object	Conditional	Typed modify payload for a `WSMessage` envelope
`grpc_start`	object	Conditional	Typed modify payload for a `GRPCStartMessage` envelope
`grpc_data`	object	Conditional	Typed modify payload for a `GRPCDataMessage` envelope
`raw`	object	Conditional	Typed modify payload for a `RawMessage` envelope

params¶

Field	Type	Required	Description
`intercept_id`	string	Yes	ID of the held envelope
`mode`	string	No	Forwarding mode: `"structured"` (default) routes through the typed dispatch; `"raw"` expects `raw_override_base64` and forwards a synthetic `RawMessage` envelope verbatim
`raw_override_base64`	string	Conditional	Base64-encoded raw bytes for raw-mode forwarding (max 10 MiB). Required when `mode` is `"raw"` and the held envelope is non-Raw

Actions¶

release¶

Forward the held envelope as-is.

modify_and_forward¶

Apply a typed modify payload (matching the held envelope's Message type) and forward the result. The typed modify schemas are described below. With mode="raw" plus raw_override_base64, the proxy builds a synthetic RawMessage envelope from the supplied bytes and forwards them verbatim regardless of the original protocol.

drop¶

Discard the held envelope and unblock the pipeline.

Typed modify payloads¶

Headers and metadata are always ordered arrays of {name, value} objects. Map shapes are rejected to preserve wire fidelity (RFC-001 §3.1: no normalization).

http¶

For HTTPMessage envelopes. Pointer-style fields (*string, *int, *bool) distinguish "field omitted" from "field set to zero value"; an omitted field leaves the held envelope's field untouched.

Field	Type	Description
`method`	string	HTTP method override (request side)
`scheme`	string	Scheme override (request side)
`authority`	string	Authority override (request side)
`path`	string	Request path override
`raw_query`	string	Raw query string override
`status`	integer	HTTP status code override (response side)
`status_reason`	string	HTTP/1.x status reason phrase override
`headers`	array	Ordered header list replacement
`trailers`	array	Ordered trailer list replacement
`body`	string	Body replacement (`text` or `base64` per `body_encoding`)
`body_encoding`	string	`"text"` or `"base64"`
`body_patches`	array	Body patches applied on top of the body replacement
`auto_content_length`	boolean	Auto-sync `Content-Length` on body change. Default `true`; set `false` to preserve CL/TE for smuggling tests

ws¶

For WSMessage envelopes.

Field	Type	Description
`opcode`	string or integer	Opcode name (`text`, `binary`, `close`, `ping`, `pong`, `continuation`) or numeric opcode in `[0, 15]`
`fin`	boolean	FIN bit override
`payload`	string	Frame payload (`text` or `base64` per `body_encoding`)
`body_encoding`	string	`"text"` or `"base64"`
`close_code`	integer	RFC 6455 status code (Close frames only)
`close_reason`	string	Close reason text (Close frames only)

grpc_start¶

For GRPCStartMessage envelopes (HEADERS frame opening one side of an RPC).

Field	Type	Description
`service`	string	gRPC service name override
`method`	string	gRPC method name override
`encoding`	string	`grpc-encoding` override
`metadata`	array	Ordered metadata list replacement (transport pseudo-headers excluded)

Trailers belong to a distinct GRPCEndMessage envelope and are out of scope for grpc_start.

grpc_data¶

For GRPCDataMessage envelopes (one length-prefixed message on a gRPC stream).

Field	Type	Description
`payload`	string	Decompressed gRPC payload (`text` or `base64` per `payload_encoding`)
`payload_encoding`	string	`"text"` or `"base64"`
`compressed`	boolean	Set the compression bit in the LPM prefix
`end_stream`	boolean	`END_STREAM` flag on the carrying H2 DATA frame

raw¶

For RawMessage envelopes. bytes_override and patches are mutually exclusive.

Field	Type	Description
`bytes_override`	string	Replacement bytes (`text` or `base64` per `bytes_encoding`)
`bytes_encoding`	string	`"text"` or `"base64"`
`patches`	array	Byte-level patches applied to the held envelope's `RawMessage.Bytes`

Response¶

Field	Type	Description
`intercept_id`	string	ID of the held envelope
`action`	string	Action performed
`status`	string	Result status (`"released"`, `"forwarded"`, `"dropped"`)
`protocol`	string	Held envelope's message-type discriminator (`"http"`, `"websocket"`, `"grpc_start"`, `"grpc_data"`, `"grpc_end"`, `"raw"`)
`direction`	string	Envelope direction (`"send"`, `"receive"`)
`matched_rules`	string[]	Rule names that fired to hold the envelope (when present)
`flow_id`	string	Flow id of the held envelope (when present)
`stream_id`	string	Stream id of the held envelope (when present)

Examples¶

Release a held HTTP request¶

// intercept
{
  "action": "release",
  "params": {"intercept_id": "int-abc-123"}
}

Modify and forward an HTTP request¶

// intercept
{
  "action": "modify_and_forward",
  "params": {"intercept_id": "int-abc-123"},
  "http": {
    "method": "POST",
    "headers": [
      {"name": "Authorization", "value": "Bearer injected-token"}
    ],
    "body": "{\"role\":\"admin\"}"
  }
}

Modify and forward a WebSocket frame¶

// intercept
{
  "action": "modify_and_forward",
  "params": {"intercept_id": "int-ws-456"},
  "ws": {
    "opcode": "text",
    "payload": "{\"action\":\"admin\"}"
  }
}

Patch a gRPC payload¶

// intercept
{
  "action": "modify_and_forward",
  "params": {"intercept_id": "int-grpc-789"},
  "grpc_data": {
    "payload": "AAAAAAk=",
    "payload_encoding": "base64"
  }
}

Forward synthetic raw bytes (smuggling test)¶

// intercept
{
  "action": "modify_and_forward",
  "params": {
    "intercept_id": "int-abc-123",
    "mode": "raw",
    "raw_override_base64": "R0VUIC8gSFRUUC8xLjENCkhvc3Q6IGV4YW1wbGUuY29tDQoNCg=="
  }
}

Drop a held request¶

// intercept
{
  "action": "drop",
  "params": {"intercept_id": "int-abc-123"}
}

Hold-window timeouts¶

Each held envelope is subject to a hold-timeout configured on the intercept queue. Defaults:

Protocol	Default hold-timeout
`http`	300 000 ms (5 min)
`ws`	60 000 ms
`sse`	60 000 ms
`grpc` / `grpc-web`	60 000 ms
`raw`, `tls-handshake`	inherit global (300 000 ms)

While a WebSocket frame is held, the proxy injects synthetic keepalive pings to the upstream so it does not idle out during human review (USK-854). Configure per-protocol overrides via configure.intercept_queue.protocol_overrides -- see configure.

Intercept -- Intercept feature guide
configure -- Configure intercept rules and queue settings
query -- Query the intercept queue with {"resource": "intercept_queue"}