---
id: review-001-forward-response-fidelity
name: Upstream response decode — vendored JSON, size caps, loud auth errors, error bodies (FWD-07, FWD-08, FWD-10, FWD-12)
status: completed
depends_on: []
scope: moderate
risk: medium
impact: component
level: implementation
tags: [adapters, review-001]
---

## Description

Review 001 findings on `src/adapters/forward.rs`'s response-decode paths
(`forward`, `forward_stream`), minus the SSE parser (its own task):

- **FWD-07**: `content_type.contains("application/json")`
  (`forward.rs:236`) misses `application/vnd.api+json`,
  `application/problem+json`, etc. → vendor JSON decoded as a `Value::Array`
  of one `Number` per byte. No response size limit on any read path
  (`json()/text()/bytes()`) — a hostile upstream controls caller memory.
  `forward_stream` never checks content-type (200 HTML → empty stream, no
  error). Fix: mime-essence matching (`application/.*+json` suffix
  semantics), response size caps on all read paths, and a loud error for
  non-SSE content on a Sub op.
- **FWD-08** (`forward.rs:104-126`, `:87-94`): invalid credential values
  (control characters, typo'd header name) silently drop the header —
  the request goes out unauthenticated and the caller only sees the
  eventual upstream 401. Fail loudly at call time. (No leak risk today —
  nothing is logged; keep it that way.)
- **FWD-10** (`forward.rs:215-227`, `:333-346`): upstream error bodies
  discarded — surfaced message is only `"HTTP {status}: {reason}"`;
  upstream diagnostics never reach the caller and unconsumed bodies
  hinder connection reuse. Surface a bounded error-body echo on the error
  path; keep the `HTTP_<status>` mapping as is (correct).
- **FWD-12**: byte-identical duplicates `value_to_path_segment`/
  `value_to_query` (`:133-151`); `forward`'s `op_type` parameter only
  toggles `ACCEPT` and its `text/` branch would buffer an entire SSE
  stream if a Sub were ever routed here (currently unreachable, inviting
  misuse — remove or make safe); the `unwrap_or_else(|_| "null")` at
  `:197` masks an unreachable serialization failure with a null body;
  `SharedHttpClient::reload` performs two separate `ArcSwap::store` calls.

## Acceptance Criteria

- [x] `application/vnd.api+json` (and `application/problem+json`) decode as JSON, not byte arrays (test)
- [x] Response size cap enforced on JSON/text/binary read paths (test)
- [x] Non-SSE content-type on a Sub op produces a loud error, not an empty stream (test)
- [x] Invalid credential values fail loudly at call time (test), no credential material logged
- [x] Upstream 4xx/5xx bodies surface bounded in the error envelope (test)
- [x] Duplicate `value_to_*` helpers collapsed; `text/` buffering branch removed or safe; reload atomic
- [x] `forward.rs` COV-01 uncovered regions (binary branch, auth arms, non-2xx mapping, content-type branches) substantially covered
- [x] `cargo test` and `cargo clippy --all-targets -- -D warnings` pass

## References

- docs/reviews/001-initial-implementation-review.md (Part D, FWD-07, FWD-08, FWD-10, FWD-12; Part I, COV-01)

## Notes

> Agent fills during implementation. Shares forward.rs with
> review-001-forward-url-safety (request side) and
> review-001-sse-parser — sequence those first or coordinate.

FWD-07: JSON dispatch is now mime-essence based
(`is_json_content_type`: type `application`, subtype `json` or `+json`
suffix; hand-rolled — `mime` is in the lock file only as a transitive
dep of reqwest, not a direct dependency, so no new crate). Read paths
(JSON/text/binary) are byte-capped by `read_body_capped` at
`RESPONSE_BODY_CAP` = 16 MiB; over-cap surfaces `HTTP_413`.
`forward_stream` checks the `Content-Type` essence (`text/event-stream`)
and a non-SSE 2xx yields one `INVALID_RESPONSE_TYPE` error envelope
instead of an empty stream. FWD-08: every silently-dropped header —
default headers (`build_request`), and the Bearer/ApiKey/Basic
credential arms — now returns a loud `CallError::internal` ("refusing
to send the request unauthenticated"); error text names the header or
namespace but never echoes credential material (tested). FWD-10:
non-2xx in both `forward` and `forward_stream` goes through
`error_envelope`: `HTTP_<status>` mapping unchanged (ADR-023) plus a
bounded echo of the upstream body — read once, capped at
`ERROR_BODY_ECHO_CAP` = 4 KiB, control characters stripped (newline/tab
kept), `[truncated]` marker when the read hit `STATUS_BODY_DRAIN`
(64 KiB) without EOF, in which case the connection is dropped rather
than drained further (bounded connection-reuse trade). FWD-12:
`value_to_path_segment`/`value_to_query` collapsed onto one
`scalar_value_to_string` extractor (the raw scalar is shared; path
rendering applies `PATH_VALUE_ENCODE_SET`, query emission stays raw for
`query_pairs_mut` — they were byte-identical pre-FWD-01 but the roles
diverged, so a single raw-then-encode function was wrong for the query
arm and the pre-existing `+`-encoding test proved it); `forward` lost
its `op_type` parameter (the Sub branch only toggled ACCEPT and the
`text/` buffering branch is gone — Sub ops route through
`forward_stream`); request-body serialization failure is a loud
`CallError::internal` instead of a silent `null` body (both handlers);
`SharedHttpClient` now holds client+config in one
`ArcSwap<SharedHttpInner>` so `reload` swaps both in a single atomic
store (minimal touch to `src/client/http_client.rs`, public API
`client()`/`config()` unchanged). COV-01 regions covered: vendor/`+json`
content-type branches, binary branch, all three auth arms (loud error
paths + the pre-existing success tests), non-2xx mapping with error-body
echo (2/404/429/500, truncated and exact), cap trips on all three read
paths, non-SSE Sub stream, SSE happy path through `forward_stream`.

## Summary

FWD-07/08/10/12 remediated in `src/adapters/forward.rs` (+
one-spot `SharedHttpClient::reload` atomicity fix in
`src/client/http_client.rs`): vendor-JSON mime-essence decode, 16 MiB
`RESPONSE_BODY_CAP` on all buffered reads (`HTTP_413` on breach),
loud `INVALID_RESPONSE_TYPE` on non-SSE Sub-op streams, loud call-time
errors for invalid credential/default-header values (no secret echo),
bounded 4 KiB error-body echo on non-2xx with a 64 KiB
`STATUS_BODY_DRAIN` read cap, duplicate scalar helpers collapsed,
`forward`'s dead `op_type` Sub branch removed, serialization failure
made loud, and the hot-reload swap made atomic (joint client+config
holder). 23 tests in `forward.rs::tests` (was 9) covering the previously
uncovered binary/auth/non-2xx/content-type regions (COV-01); full
suite 265 passing. Commits 9bb8487 (core) and 7ac2dc5 (tests).