Thanks Prashant, I’m supportive of introducing a generic client capabilities header. I agree with the direction of treating this primarily as capability advertisement for compatibility and response shaping rather than as a trust mechanism.
I left a couple comments on the PR. One thing I’m wondering is whether we should discuss and define the versioning and compatibility model for capability header values as part of this proposal. Capabilities like `read-restrictions` seem very likely to evolve over time in ways that older clients may not be able to safely consume. I’m curious whether the community thinks capability values should represent versioned compatibility contracts (like read-restrictions.v1) and whether we want to define any expectations around cross-version compatibility behavior now as we introduce the header. Sung On 2026/05/18 18:51:52 Prashant Singh wrote: > Hi all, > > I'd like to propose adding a new HTTP header, > X-Iceberg-Client-Capabilities, > to the REST catalog spec. This was brought up at the Read Restrictions > community sync on May 12, 2026; I'm bringing it to the broader list now > for > wider feedback. > > PR: https://github.com/apache/iceberg/pull/16394 > Recording: https://youtu.be/b9p6mI-k-0I > > *Context* > > Iceberg's REST protocol keeps evolving — server-side scan > planning, remote signing, and the in-flight ReadRestrictions proposal > (PR #13879) are all features that catalogs may return but older clients > can't handle. Today there's no standard way for a client to declare > "I understand X" to the server. One direction discussed at the community > sync was to introduce a single generic capability header rather than > per-feature negotiations; this thread is to gather broader input on that > proposal. > > *Proposal* > > Send X-Iceberg-Client-Capabilities on every catalog request: > > ex: X-Iceberg-Client-Capabilities: > vended-credentials,remote-signing,scan-planning > > The Java SDK adds it via HTTPClient.baseHeaders — the same path used for > X-Client-Version and X-Client-Git-Commit-Short today. Other client > implementations (PyIceberg, Rust, Go, etc.) can mirror the same enum. > > Initial capability set for Java SDK: > - vended-credentials > - remote-signing > - scan-planning > > read-restrictions will be added once PR #13879 lands. > > > * Design notes worth feedback* > 1. Forward-compat hint, not security. The header is informational — > clients > can trivially spoof its value, so servers MUST NOT base trust or > authorization decisions on it. Trust establishment (mTLS, OAuth, etc.) > is out of scope. The spec parameter description states this explicitly. > > 2. enum: constraint on the schema. Mirrors X-Iceberg-Access-Delegation. > Adding > a new capability is a one-line spec edit; generated clients can > validate > values; servers get a machine-readable closed list of recognized > values. > > 3. Skipped for /v1/oauth/tokens. OAuth token endpoints may be served by > external IdPs (Okta, Auth0, etc.) where Iceberg-specific headers are > noise. Mirrors how X-Iceberg-Access-Delegation is handled. > > *Links* > > - PR: https://github.com/apache/iceberg/pull/16394 > - May 12 sync recording: https://youtu.be/b9p6mI-k-0I > - Related: PR #13879 <https://github.com/apache/iceberg/pull/13879> > > Thanks, > Prashant >
