[Tough-Respawn]

Overview

Fix the full re-prefill after slot save → restore for SWA and hybrid/recurrent models (e.g. Qwen3.5 / Qwen3-Next).

When a slot is restored, the server rebuilds the token cache but not the context checkpoints (slot->prompt.clear() discards them, and they are not part of the save file). SWA and hybrid/recurrent models cannot rewind their state without a checkpoint, so the next divergent prompt triggers "forcing full prompt re-processing due to lack of cache data" even when it shares a long prefix with the saved state. Since a recurrent state cannot be rewound, checkpoints are not reconstructible after the fact, they have to travel with the save file.

This PR preserves the context checkpoints inside the slot save file:

• save_slot_checkpoints() appends a tagged payload (SCKP magic + version + count + per-checkpoint pos fields and state blobs) after the llama state payload
• load_slot_checkpoints() reads it back at the offset returned by llama_state_seq_load_file and reattaches the checkpoints to the slot
• Backward/forward compatible: old files restore exactly as before (no magic → silent skip), and new files load fine on older servers (they stop reading at the end of their own payload)
• Guards against damaged files: blob sizes are checked against the remaining file size before allocating, truncation → warning and clean abort (no partial state), and a restored checkpoint that fails to load falls back to full re-processing
• id_task is not serialized and is reset to -1 on load (task ids are not meaningful across save/restore)
• n_written/n_read include the appended checkpoint bytes, so the reported size matches the file on disk; no appendix is written when the slot holds no checkpoints, leaving those files byte-identical to the format without this feature
• A draft context configured differently at restore no longer aborts the server: the draft checkpoint data is dropped instead (second commit, see below)

The implementation lives in tools/server/server-context.cpp, using std::ifstream/std::ofstream with small read/write helpers.

Independently reproduced and confirmed by 10 users across CUDA, Metal, Vulkan and an ollama integration, including a cross-machine CUDA → Metal restore. Details below.

Additional information

Tests included in tools/server/tests/unit/test_slot_save.py:

test_slot_restore_preserves_context_checkpoints:

1. Send a long prompt (reference: a divergent re-prompt on the live slot rolls back to a checkpoint → partial reuse, n_live tokens processed)
2. save the slot
3. Overwrite the slot with an unrelated prompt
4. restore the slot
5. Send the same divergent re-prompt, the test asserts prompt_n == n_live

Without the fix, step 5 re-processes the full prompt. The fixture sets cache_ram = 0 (the host prompt cache would otherwise mask the path under test) and n_ubatch = 32 (so a prompt-processing checkpoint lands before the divergence point).

test_slot_restore_damaged_checkpoint_appendix damages the appendix (oversized blob, corrupted state, 1025 entries) and checks that the server stays up and falls back or reuses as expected. The first test also runs with a non-ASCII file name.

test_slot_restore_checkpoints_draft_kv_type_change runs the same scenario with a draft model, and restarts the server between save and restore with the same draft KV cache type (control) or a different one (q8_0). The restored request must succeed and still roll back to the checkpoint.

The first test passes locally against this branch after the rebase, and the same scenario fails on master (assert 210 == 31). The file now also carries the multimodal slot tests that came in with #26640.

Validated locally on 06d9d0f with the full CPU CI (ci/run.sh): exit 0, ctest 46/46 (debug) and 49/49 (release), 6/6 with model in both. tools/server/tests/unit/test_slot_save.py: 10/10.

On 1bfe730 (Linux, CPU): tools/server/tests/unit/test_slot_save.py 12/12, and the new draft test fails without the second commit. The full ci/run.sh was not re-run on 1bfe730.

On 9f098d9 (Linux, CPU, merged with master 8345f33): test_slot_save.py 16/16, the damaged-appendix cases fail without the third commit. Also checked on Windows (peak RSS 19.7 MiB with @junmo-kim's fixture, 83.8 MiB before) and with hybrid models + draft-mtp / draft-eagle3 across a process restart (same tokens re-processed and same output as the live slot). The full ci/run.sh was not re-run.

On 9bc4fa7 (merged with master 42b021b): test_slot_save.py 17/17.

Rebase note. #26640 ("server : support slot save/restore with media inputs") reworked the same SLOT_SAVE / SLOT_RESTORE handlers: the save now writes a packed server_tokens::serialize() payload including media chunks instead of get_text_tokens(), and the restore does a two-pass llama_state_seq_load_file + deserialize() + validate(). This PR was rebased onto master and flattened to a single commit (06d9d0f); the later commits below came later. The split is unchanged: the state payload belongs to master, the checkpoint appendix belongs to this PR, it is still appended after the payload, and the load offset is still the end of the payload returned by llama_state_seq_load_file.

Second commit (1bfe730): draft KV type mismatch. Fix by @okigan, taken as-is, plus a regression test. The checkpoints also carry the draft model's state, and nothing checked it against the running draft context: restoring a slot saved with -ctkd f16 into a server started with -ctkd q8_0 aborted in load_dft() with checkpoint size mismatch. On restore, one draft checkpoint is now test-loaded; if it does not fit, the draft data is dropped with a warning instead of crashing. The target still rolls back to its checkpoint and the output is unchanged; the draft restarts without the prefix for that session.

Third commit (9f098d9): appendix hardening. Reported by @junmo-kim: a blob was allocated before checking that its bytes are in the file. Blob sizes are now checked against the remaining file size, as llama_state_seq_load_file does, and the 16 GiB cap is gone. Same commit:

• a restored checkpoint that fails to load falls back to full re-processing instead of aborting
• non-ASCII file names on Windows: the file is opened with std::filesystem::u8path(), as in common : remove fs_open_ifstream() by using u8path() #29841
• the 1024 count cap is replaced by keeping the last n_ctx_checkpoints while reading

The file format does not change.

Fourth commit (2b81c0a): a slot save whose checkpoint appendix cannot be written completely now returns an error, like a failed payload write, instead of reporting success.

Fifth commit (9bc4fa7): a checkpoint with an empty target blob is rejected when loading the appendix (found by the automated review); logging carries the slot id, and the draft test model is loaded from the HF cache.

Sixth commit (2cb56aa): load_tgt / load_dft return bool; a checkpoint restored from a slot file that fails to load falls back to full re-processing, a checkpoint created in memory still aborts (requested in review).

Fixes #25913, same root cause reported there (checkpoints never persisted, cleared on restore).

Related: #22384 fixed the checkpoint lookup for recurrent models in a live slot; this PR addresses the save/restore path, which loses the checkpoints entirely.

Prior attempts: #20819 and #24028 address the same gap with a separate companion file (.checkpoints / .ckpt). I only found them after opening this PR. Both currently conflict with master (#20819 still uses the removed server_prompt_checkpoint type, #24028 predates the #26640 handler rework), neither includes a regression test, and a companion file can drift out of sync with its state file (in #20819 a later save without checkpoints leaves the old companion in place). This PR keeps everything in a single file instead. #28074 was closed by its author as a duplicate of this PR.

User-reported results:

Reporter	Date	Setup	Result
wagi-sho	Jul 23	Vulkan, Radeon 8050S (gfx1151), Qwen3.5-35B-A3B (Gated DeltaNet)	With the fix: 181.9 s → 4.7 s (38.7x); 58,202 → 516 tokens
terisuke	Jul 25	Cross-machine: CUDA GB10 → Metal M3 Ultra	With the fix: cache_n 0 → 310; identical output verified by SHA-256
kfiramar	Aug 14	Qwen3.8	Confirmed; no measurements provided
YUXUANCHENG	Aug 16	Not specified	Confirmed; no measurements provided
yitizi	Aug 31	Qwen3.8-27B, hybrid SSM, strict-prefix extension	Bug on master: 8 tokens processed without a restore in between vs 2,005 with one
dgabehar	Sep 08	Vulkan, Radeon 780M (gfx1103), Qwen3.8-27B + draft-MTP	With the fix: ~71 s → ~3 s (23.7x) on a 4,500-token prefix
m4xw	Sep 15	Ollama integration, Anthropic-style caching	With the fix: avoided the full prefill after a process restart
ReplicantBob	Sep 22	Windows, CUDA 13.3, RTX 5070 Ti, Qwen3.6-35B-A3B hybrid, q8_0 KV	With the fix: erase → restore 4,460 → 30 tokens (149x); a 36k-token agent session resumed with 13 tokens re-processed
ChrisC381	Sep 27	Radeon 8060S (gfx1151), Qwen3.8-Flash-Next 112 GB hybrid, shared-MTP	With the fix: prefill 2,991 ms → 109 ms (27x); prompt_n 2,337 → 4; byte-identical greedy output; slot-save suite 10/10
okigan	Oct 04	Apple M5 Pro, Metal, Qwen3.8-27B (hybrid) and gemma-4-12b (SWA), f16 / q8_0 / q4_0 KV	With the fix: prompt reuse after save → restart → restore 0 % → 99 % (64 turns); resume 6.07 s → 0.79 s (Qwen3.8-27B, f16)

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES — the implementation and the test were primarily AI-generated (Claude Fable 5), working under my direction from a bug I found and measured on my hardware. I reviewed the code, ran the repro and the tests on real models (SWA and hybrid), and validated the fix end-to-end. This description was also drafted with AI assistance.

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[wagi-sho]

Independent verification on Vulkan / gfx1151 with a Gated DeltaNet hybrid — the fix works

Built this branch and measured it against master on a third backend/architecture. Summary: 181.9s → 4.7s on the first request after a restore.

Environment

• Nex-N2-mini (Qwen3.5-35B-A3B, Gated DeltaNet hybrid), UD-Q4_K_XL
• Windows, Vulkan, AMD Radeon 8050S (gfx1151), 64GB unified memory
• Server: -c 131072 --slot-save-path <dir> --jinja -np 1 --ctx-checkpoints 128 --checkpoint-min-step 128 --slot-prompt-similarity 0.1 --cache-ram 0 -lv 4
• Controlled comparison: this PR (866543040) vs b10091 (b4d6c7d8f), which differs by only two unrelated commits. Same prompt, same flags, full server restart between every measurement.

Cold prefill (baseline, identical on both) — 58,202 tokens, ~185s, save 58,209 tokens.

First request after restore (same 58K prefix, divergent tail)

build	prompt_n	cache_n	wall
b10091 (master)	58,202	0	181.9s
this PR, no id_slot	516	57,686	4.7s
this PR, with id_slot	516	57,686	4.7s

The 516 tokens are the expected checkpoint-granularity rollback, not a full-prefix hit — matching the caveat in #25913.

Multi-turn after restore (conversation accumulates, 10 turns)

build	turn 1	turns 2-10
b10091 (master)	58,210 tok / 181.4s	15-52 tok / 2.5s
this PR	524 tok / 5.5s	31-52 tok / 3.2s

cache_n grows monotonically (57,686 → 58,548) across turns. No #22450-style hang or checkpoint eviction loop over 10 turns. The difference between the two builds is confined to the first request after restore, which is exactly the scope this PR claims.

Output sanity — the dangerous failure mode here is a fast but corrupted answer, so I checked the content, not just the timings. After restore the model still reads the 58K prompt body correctly: "does rule 5 come before rule 9?" → Yes; "is rule 2999 present?" → Yes; "which rule number appears first?" → 0; "is this policy sorted?" → Yes. No sign of a corrupted recurrent state.

One note on the test fixture — setting cache_ram = 0 is essential and I want to underline it, because I lost a day to exactly this before finding #25913. A 5K restore appeared to succeed (16 tokens processed) until I restarted the server to clear cache-ram; the same restore then reprocessed 5,428 tokens, a 340x difference. Leftover checkpoints from a previous live session silently mask the broken on-disk path. Good that the fixture already handles it.

Happy to re-run any of this on Vulkan/gfx1151 if it helps review.

[terisuke]

First of all, thanks to @wagi-sho for pointing out the --cache-ram 0 issue and for sharing the much larger Vulkan test. That caught a mistake in my first run and saved me from reporting a false positive.

I tried something slightly different: restoring a slot file produced on a CUDA machine (GB10) on a different machine running the Metal backend (M3 Ultra).

Both servers used --cache-ram 0, the source server was restarted before saving, and I verified that both machines were using the exact same model file and the same transferred slot file (SHA-256 checked on both sides).

On the Metal machine:

Run	cache_n	prompt_n	Content SHA-256
Control (no restore)	0	314	50afb036...414ff8
After restoring CUDA slot	310	4	acf1bbed...af15f

The original CUDA run also produced acf1bbed...af15f.

The interesting part is that Metal does not normally generate the same completion as CUDA for this prompt. Without restoring the slot, the Metal output hashes to 50afb036...; after restoring the CUDA-generated state it matches the CUDA output instead. That makes me think the restored state is actually being reused across both the backend boundary and the machine boundary.

This is only a small portability test (~314 prompt tokens, a single restore, and a single completion), so I wouldn't draw any conclusions about long contexts or repeated restores yet.

One other thing I noticed while testing: the save response reported:

n_written = 179,512,968

but the actual slot file written to disk was:

336,407,384 bytes

The server log says one context checkpoint was appended, so the extra ~149.6 MiB appears to be that checkpoint payload. It might be worth documenting that n_written currently reflects only the base llama-state payload, or exposing the total on-disk size separately to avoid confusing callers.

Thanks again for working on this PR. If it would be useful, I can also test longer CUDA → Metal restores or the reverse Metal → CUDA direction.

[Tough-Respawn]

Thanks @wagi-sho @terisuke for digging into this, and especially for the cross-machine test — that's a case I had no way to cover locally.

On CUDA -> Metal: the result is expected rather than lucky. The checkpoint blobs come from llama_state_seq_get_data_ext, the same serializer that writes the base payload, just with the PARTIAL_ONLY flag — so the appendix is portable exactly to the degree the base payload is. Related caveat I hit while testing: save files are only
comparable byte-for-byte within one backend (a Vulkan and a CPU build of the same model, prompt and seed produce different bytes; -ngl 0 makes them match), which is an independent confirmation of what you saw.

On n_written: you're right, and it's a bug rather than something to document. Before this PR llama_state_seq_save_file was the only writer and returned file.tell() at close, so n_written was the on-disk size by construction. This PR appends the checkpoints after that file is closed, and reuses the offset returned by the loader to find them — which is what makes the format backward compatible — but I never added the appended bytes back into the response. n_read has the same blind spot, and the appendix header was written even for slots with no checkpoints, so every save file under-reported, not only SWA/hybrid ones.

Your numbers match the format exactly: 336,407,384 - 179,512,968 = 156,894,416 = 60 bytes of framing (12-byte header, then 8 + 4 + 4 and three 8-byte length prefixes) plus a 149.62 MiB blob, i.e. the single checkpoint your log reported.

I fixed the accounting rather than documenting the gap, because a caller that trusts n_written to copy or verify a file truncates the appendix — and the loader tolerates a truncated appendix by warning, dropping the checkpoints and restoring anyway. The restore returns 200 and looks fine while silently losing what this PR adds.

Follow-up commit:

• save_slot_checkpoints and load_slot_checkpoints return the bytes
  appended/consumed and the responses add them, so n_written and n_read are the
  on-disk size again;
• no appendix is written at all when the slot holds no checkpoints, so those files
  stay byte-identical to the format without this feature;
• the tests assert n_written/n_read == os.path.getsize(save_file) on both paths.
  That guard was missing, which is why this slipped through.

Measured on stories260K (full attention) and tinygemma3 (SWA):

	reported	on disk
before, no checkpoints	23,768	23,780	the 12-byte empty appendix
before, SWA appendix	3,528,824	9,096,332	61% of the file unreported
after, no checkpoints	23,768	23,768	payload byte-identical, tail gone
after, SWA appendix	9,096,332	9,096,332	save and restore

Files already written by this branch keep working: one carrying an empty appendix now reports its real size, and an older SWA save still drives the same checkpoint rollback as a live slot (31 prompt tokens instead of 212).

On more testing: yes please, and a divergent follow-up would be worth more than a longer one. Your test restored and continued straight ahead, which works even without checkpoints; the case this PR fixes is a prompt that diverges mid-state after the restore. Comparing prompt_n between a live slot and a restored one on that path, in either direction, would tell us whether restored checkpoints are usable across the backend boundary and not just loadable

[whoreson]

Could have simply copied over that ~10 lines of code from a certain fork (which has had this elementary feature for probably a year now) instead of slopping away.

[Tough-Respawn]

Could have simply copied over that ~10 lines of code from a certain fork (which has had this elementary feature for probably a year now) instead of slopping away.

Which fork, which file/commit? The two obvious ones (ik_llama.cpp #1762, koboldcpp) either have the same bug open or use an in-RAM mechanism, not on-disk checkpoint persistence for recurrent state. Happy to compare.

[whoreson]

Yes the

verboten

fork.

And even PicoLM has it.

[Tough-Respawn]

Yes the

verboten

fork.

And even PicoLM has it.

Bold to say "10 lines" and "slop" in the same sentence where you point at picolm, because I actually read picolm: the on-disk checkpoint save/restore in your own server is ~217 lines (server.c 1143-1184 + 1242-1416), before the bookkeeping and the SSM (de)serialization. Off by ~20x on code you wrote yourself is a rough way to argue it's trivial.

As for the "verboten" fork you keep not naming: I can't compare against something you won't link. Name it, or it isn't really an argument.

And picolm is a separate 2.5k-line engine for a handful of models. This is llama.cpp, where the checkpoints get dropped on restore today (#25913, 181.9s on master above). #17428 was your live-slot half; this is the disk half. If it's really 10 lines, show the commit.

I'm here to contribute, not to troll or be trolled.

erlaubt

[whoreson]

Fus
Roh
ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_1FAEFB6177B4672DEE07F9D3AFC62588CCD2631EDCF22E8CCC1FB35B501C9C86

[Tough-Respawn]

Fus Roh ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_1FAEFB6177B4672DEE07F9D3AFC62588CCD2631EDCF22E8CCC1FB35B501C9C86

It's Fus Ro Dah, three words. You dropped the last one, same way you dropped ~200 lines counting picolm. Come back with a shout that's finished and a commit that exists ;)

[kfiramar]

This is a real and important fix, I've encountered it just now - trying to use qwen 3.8 and not being able to restore anything...

Really hope it is merged ASAP

[Tough-Respawn]

Rebased onto master to resolve the conflict introduced by #26640, which reworked the same SLOT_SAVE / SLOT_RESTORE handlers this PR touches.

Master now writes a packed server_tokens::serialize() payload instead of a plain text-token list, and restores it through a two-pass llama_state_seq_load_file followed by deserialize() and validate(). That payload is master's, so I kept it untouched. This PR only adds a checkpoint appendix after it, which stays orthogonal: llama_state_seq_load_file still returns the end offset of the payload, which is exactly where the appendix starts, so media save/restore is unaffected.

The appendix is written after llama_state_seq_save_file has already closed the file, so it is counted separately in n_written / n_read. Counting it on both sides also keeps master's own n_read == n_written assertion in test_slot_save_restore_with_image valid.

The two original commits are squashed into one, and the now-dead if (nwrite > 0) guard was dropped since master already bails out on nwrite == 0. Net change against master: 195 insertions, 2 deletions.

[YUXUANCHENG]

Same issue… hope this gets merged asap.

[yitizi]

Independent confirmation of this bug, from a different model and a different angle.

I run a local proxy in front of llama-server that manages slots for multi-session agent work: on a session switch it saves the outgoing session's slot and restores the incoming one via /slots. The whole point was to avoid cold prefill on a hybrid model. It didn't work — every restored session still re-processed its full prompt, while the restore itself reported success.

Reduced to a single variable on Qwen3.8-27B (hybrid SSM), via /completions:

feed a 2001-token prompt → evict the slot with unrelated content
→ send a strict extension (same 2001-token prefix + a few tokens)

  without restore in between:  8 tokens processed
  with    restore in between:  2005 tokens processed   (full recompute)

Root cause matches your analysis exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only restores tokens, and the reuse path then takes do_reset because the checkpoint list is empty.

Two things that may be worth adding to the picture:

id_slot pinning bypasses the host prompt cache. Same 3211-token prompt after evicting the slot: pinned to a slot, 0 reused; unpinned, 3207 reused. The cache lookup happens during slot selection. Anyone using /slots save/restore is likely pinning too, so they lose both paths at once.

In my case, not using save/restore at all turned out to be faster than fixing it. With the proxy's slot management disabled, a new agent session went from 14,199 tokens re-prefilled (22.5 s) to 4 tokens (0.45 s), because the host prompt cache already handles cross-session prefix reuse correctly. That doesn't reduce the value of this fix — explicit session persistence through /slots still needs it — but the two mechanisms interact.

I had built a local fix for the same problem before finding this PR (a separate .ckpt sidecar file). Yours is the better design — single file, compatible in both directions — so I'm not opening anything competing.

AI usage disclosure: the measurements and root-cause analysis were done with Claude assisting; I reproduced and verified everything on my own hardware. This comment is my own.

[Tough-Respawn]

Independent confirmation of this bug, from a different model and a different angle.

I run a local proxy in front of llama-server that manages slots for multi-session agent work: on a session switch it saves the outgoing session's slot and restores the incoming one via /slots. The whole point was to avoid cold prefill on a hybrid model. It didn't work — every restored session still re-processed its full prompt, while the restore itself reported success.

Reduced to a single variable on Qwen3.8-27B (hybrid SSM), via /completions:

feed a 2001-token prompt → evict the slot with unrelated content
→ send a strict extension (same 2001-token prefix + a few tokens)

  without restore in between:  8 tokens processed
  with    restore in between:  2005 tokens processed   (full recompute)

Root cause matches your analysis exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only restores tokens, and the reuse path then takes do_reset because the checkpoint list is empty.

Two things that may be worth adding to the picture:

id_slot pinning bypasses the host prompt cache. Same 3211-token prompt after evicting the slot: pinned to a slot, 0 reused; unpinned, 3207 reused. The cache lookup happens during slot selection. Anyone using /slots save/restore is likely pinning too, so they lose both paths at once.

In my case, not using save/restore at all turned out to be faster than fixing it. With the proxy's slot management disabled, a new agent session went from 14,199 tokens re-prefilled (22.5 s) to 4 tokens (0.45 s), because the host prompt cache already handles cross-session prefix reuse correctly. That doesn't reduce the value of this fix — explicit session persistence through /slots still needs it — but the two mechanisms interact.

I had built a local fix for the same problem before finding this PR (a separate .ckpt sidecar file). Yours is the better design — single file, compatible in both directions — so I'm not opening anything competing.

AI usage disclosure: the measurements and root-cause analysis were done with Claude assisting; I reproduced and verified everything on my own hardware. This comment is my own.

Thanks, this is a really useful data point: different model, different path, and a clean single-variable repro (8 vs 2005 tokens). Matches the root cause exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only brings back tokens, and the reuse path then hits do_reset because the checkpoint list is empty.

On id_slot pinning bypassing the host prompt cache: good catch, and I'd treat it as a separate issue from this PR.
The cache lookup happens during slot selection, so pinning skips it regardless of checkpoints worth its own report, since anyone using /slots save/restore is likely pinning and loses both paths at once.
My PR only restores the explicit-persistence path.

And agreed on the interaction: for pure cross-session prefix reuse the host prompt cache already handles it, often faster. This fix is for explicit session persistence through /slots: state that has to survive a server restart and be addressable by name which the host cache doesn't cover. Complementary, not competing.

On the .ckpt sidecar: thanks for not opening something competing. Single file + backward/forward compatible was the deliberate choice a sidecar can desync from the state file, whereas an appendix that older servers stop reading at their own payload end can't.

[yitizi]

Independent confirmation of this bug, from a different model and a different angle.
I run a local proxy in front of llama-server that manages slots for multi-session agent work: on a session switch it saves the outgoing session's slot and restores the incoming one via /slots. The whole point was to avoid cold prefill on a hybrid model. It didn't work — every restored session still re-processed its full prompt, while the restore itself reported success.
Reduced to a single variable on Qwen3.8-27B (hybrid SSM), via /completions:

feed a 2001-token prompt → evict the slot with unrelated content
→ send a strict extension (same 2001-token prefix + a few tokens)

  without restore in between:  8 tokens processed
  with    restore in between:  2005 tokens processed   (full recompute)

Root cause matches your analysis exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only restores tokens, and the reuse path then takes do_reset because the checkpoint list is empty.
Two things that may be worth adding to the picture:
id_slot pinning bypasses the host prompt cache. Same 3211-token prompt after evicting the slot: pinned to a slot, 0 reused; unpinned, 3207 reused. The cache lookup happens during slot selection. Anyone using /slots save/restore is likely pinning too, so they lose both paths at once.
In my case, not using save/restore at all turned out to be faster than fixing it. With the proxy's slot management disabled, a new agent session went from 14,199 tokens re-prefilled (22.5 s) to 4 tokens (0.45 s), because the host prompt cache already handles cross-session prefix reuse correctly. That doesn't reduce the value of this fix — explicit session persistence through /slots still needs it — but the two mechanisms interact.
I had built a local fix for the same problem before finding this PR (a separate .ckpt sidecar file). Yours is the better design — single file, compatible in both directions — so I'm not opening anything competing.
AI usage disclosure: the measurements and root-cause analysis were done with Claude assisting; I reproduced and verified everything on my own hardware. This comment is my own.

Thanks, this is a really useful data point: different model, different path, and a clean single-variable repro (8 vs 2005 tokens). Matches the root cause exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only brings back tokens, and the reuse path then hits do_reset because the checkpoint list is empty.

On id_slot pinning bypassing the host prompt cache: good catch, and I'd treat it as a separate issue from this PR. The cache lookup happens during slot selection, so pinning skips it regardless of checkpoints worth its own report, since anyone using /slots save/restore is likely pinning and loses both paths at once. My PR only restores the explicit-persistence path.

And agreed on the interaction: for pure cross-session prefix reuse the host prompt cache already handles it, often faster. This fix is for explicit session persistence through /slots: state that has to survive a server restart and be addressable by name which the host cache doesn't cover. Complementary, not competing.

On the .ckpt sidecar: thanks for not opening something competing. Single file + backward/forward compatible was the deliberate choice a sidecar can desync from the state file, whereas an appendix that older servers stop reading at their own payload end can't.

Filed it separately as #28139, as you suggested.

One correction to what I wrote above: the trigger is narrower than I described.
It only happens when the pinned slot is empty — pinning to a slot that still
holds tokens works fine. Root cause is a 0.0f/0.0f in the f_keep computation,
which makes the f_keep < 0.5f guard false and skips the cache lookup.

[Tough-Respawn]

Independent confirmation of this bug, from a different model and a different angle.
I run a local proxy in front of llama-server that manages slots for multi-session agent work: on a session switch it saves the outgoing session's slot and restores the incoming one via /slots. The whole point was to avoid cold prefill on a hybrid model. It didn't work — every restored session still re-processed its full prompt, while the restore itself reported success.
Reduced to a single variable on Qwen3.8-27B (hybrid SSM), via /completions:

feed a 2001-token prompt → evict the slot with unrelated content
→ send a strict extension (same 2001-token prefix + a few tokens)

  without restore in between:  8 tokens processed
  with    restore in between:  2005 tokens processed   (full recompute)

Root cause matches your analysis exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only restores tokens, and the reuse path then takes do_reset because the checkpoint list is empty.
Two things that may be worth adding to the picture:
id_slot pinning bypasses the host prompt cache. Same 3211-token prompt after evicting the slot: pinned to a slot, 0 reused; unpinned, 3207 reused. The cache lookup happens during slot selection. Anyone using /slots save/restore is likely pinning too, so they lose both paths at once.
In my case, not using save/restore at all turned out to be faster than fixing it. With the proxy's slot management disabled, a new agent session went from 14,199 tokens re-prefilled (22.5 s) to 4 tokens (0.45 s), because the host prompt cache already handles cross-session prefix reuse correctly. That doesn't reduce the value of this fix — explicit session persistence through /slots still needs it — but the two mechanisms interact.
I had built a local fix for the same problem before finding this PR (a separate .ckpt sidecar file). Yours is the better design — single file, compatible in both directions — so I'm not opening anything competing.
AI usage disclosure: the measurements and root-cause analysis were done with Claude assisting; I reproduced and verified everything on my own hardware. This comment is my own.

Thanks, this is a really useful data point: different model, different path, and a clean single-variable repro (8 vs 2005 tokens). Matches the root cause exactly: server_prompt::clear() drops the checkpoints, SLOT_RESTORE only brings back tokens, and the reuse path then hits do_reset because the checkpoint list is empty.
On id_slot pinning bypassing the host prompt cache: good catch, and I'd treat it as a separate issue from this PR. The cache lookup happens during slot selection, so pinning skips it regardless of checkpoints worth its own report, since anyone using /slots save/restore is likely pinning and loses both paths at once. My PR only restores the explicit-persistence path.
And agreed on the interaction: for pure cross-session prefix reuse the host prompt cache already handles it, often faster. This fix is for explicit session persistence through /slots: state that has to survive a server restart and be addressable by name which the host cache doesn't cover. Complementary, not competing.
On the .ckpt sidecar: thanks for not opening something competing. Single file + backward/forward compatible was the deliberate choice a sidecar can desync from the state file, whereas an appendix that older servers stop reading at their own payload end can't.

Filed it separately as #28139, as you suggested.

One correction to what I wrote above: the trigger is narrower than I described. It only happens when the pinned slot is empty — pinning to a slot that still holds tokens works fine. Root cause is a 0.0f/0.0f in the f_keep computation, which makes the f_keep < 0.5f guard false and skips the cache lookup.

Thanks, and nice catch on the NaN. I'll follow #28139. Appreciate you keeping it separate from this one.

[dgabehar]

Another independent confirmation, from yet another hardware/model combination, plus one adjacent finding that's worth flagging even though it's out of this PR's scope.

Setup: Vulkan/AMD APU (Radeon 780M, gfx1103), Qwen3.8-27B with --spec-type draft-mtp (speculative decoding via a draft model), --slot-save-path, single slot (--parallel 1).

Confirmed the bug and root cause independently before finding this PR: POST /slots/0?action=restore reports success and a perfect LCP match at the next request's slot-selection step (f_sim_best = 1.000), but the reuse-validity check immediately after finds slot.prompt.checkpoints empty and forces n_past = 0. Verbose (-lv 5) logging showed the post-restore request replaying the exact same checkpoint-creation sequence as a genuinely cold prompt, byte for byte.

Confirmed this PR's fix works, hand-ported against a different pinned commit than this PR's own base (our build tracks ggml-org/llama.cpp via a pinned-commit + vendored-patch workflow rather than a live branch, so the line context differs slightly — the logic is unchanged). Real production test: save a slot mid-conversation, evict it with an unrelated request, restore, then continue the original conversation — cache_n went from 0 to the full restored token count, wall clock from ~71s to ~3s for a ~4500-token prefix.

One thing this PR doesn't cover, for anyone else hitting a similar wall: on a speculative-decoding deployment (--model-draft/--spec-type), neither SLOT_SAVE nor SLOT_RESTORE persist the draft model's own KV cache (ctx_dft) directly — only whatever data_dft happens to be attached to a checkpoint that already existed at save time. For a short conversation that hasn't hit its first checkpoint yet, that means the draft model silently restarts from empty on restore. We added a small complementary fix for this (a companion <file>.dft, using llama_state_seq_get_size_ext/get_data_ext/set_data_ext directly on ctx_dft) — happy to open it as a follow-up PR against this one once it lands, or fold it in here if that's more useful. Not blocking on this PR either way, just flagging it as adjacent scope.

Thanks for tracking this down and writing a proper regression test — hope this helps get it unstuck.

[Tough-Respawn]

Another independent confirmation, from yet another hardware/model combination, plus one adjacent finding that's worth flagging even though it's out of this PR's scope.

Setup: Vulkan/AMD APU (Radeon 780M, gfx1103), Qwen3.8-27B with --spec-type draft-mtp (speculative decoding via a draft model), --slot-save-path, single slot (--parallel 1).

Confirmed the bug and root cause independently before finding this PR: POST /slots/0?action=restore reports success and a perfect LCP match at the next request's slot-selection step (f_sim_best = 1.000), but the reuse-validity check immediately after finds slot.prompt.checkpoints empty and forces n_past = 0. Verbose (-lv 5) logging showed the post-restore request replaying the exact same checkpoint-creation sequence as a genuinely cold prompt, byte for byte.

Confirmed this PR's fix works, hand-ported against a different pinned commit than this PR's own base (our build tracks ggml-org/llama.cpp via a pinned-commit + vendored-patch workflow rather than a live branch, so the line context differs slightly — the logic is unchanged). Real production test: save a slot mid-conversation, evict it with an unrelated request, restore, then continue the original conversation — cache_n went from 0 to the full restored token count, wall clock from ~71s to ~3s for a ~4500-token prefix.

One thing this PR doesn't cover, for anyone else hitting a similar wall: on a speculative-decoding deployment (--model-draft/--spec-type), neither SLOT_SAVE nor SLOT_RESTORE persist the draft model's own KV cache (ctx_dft) directly — only whatever data_dft happens to be attached to a checkpoint that already existed at save time. For a short conversation that hasn't hit its first checkpoint yet, that means the draft model silently restarts from empty on restore. We added a small complementary fix for this (a companion <file>.dft, using llama_state_seq_get_size_ext/get_data_ext/set_data_ext directly on ctx_dft) — happy to open it as a follow-up PR against this one once it lands, or fold it in here if that's more useful. Not blocking on this PR either way, just flagging it as adjacent scope.

Thanks for tracking this down and writing a proper regression test — hope this helps get it unstuck.

Thanks, really useful confirmation on a fresh combo. Agreed the draft-context gap is real and belongs in a follow-up, #28619 is the right tracker. A couple of open suggestions for when you pick it up : the in-RAM prompt cache already saves both contexts with FLAGS_NONE, a good reference for the KV part; a separate tagged section (own version, written even without checkpoints) would keep SCKP v1 readers intact; and since draft-mtp holds state outside the KV, the meaningful test is draft acceptance after save/evict/restore versus the same continuation on a slot that stayed in memory.

[ggerganov]

@Tough-Respawn Could you rebase on latest master?

[pwilkin]

@ggerganov did it.