dsh-image-count-guard

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A fallback plugin for DSH visual-model sessions that hit an image-count limit.

The problem it solves

An upstream gateway may enforce a hard cap on the number of images in a single request (measured on OpenCode Go: 20 images). The dsh-llm-pi-ai adapter, however, budgets images purely by base64 bytes (maxRequestImageBytes, 20 MiB by default), with no budget for image count, so:

1. images accumulate in the session history until there are 36 of them (11.79 MiB raw / roughly 15.72 MiB as base64);
2. after switching to a visual model (opencode-go/deepseek-v4.1-flash), 15.72 MiB < 20 MiB, so the adapter considers the request fine and sends all 36 images inline;
3. the gateway replies 400 INVALID_REQUEST: a request may include at most 20 images;
4. the official dsh-compaction-image-offload only recognizes IMAGE_OFFLOAD_REQUIRED, not this error code → the step fails;
5. from then on every step of that session fails (sending "continue" again makes no difference) and the session is completely stuck.

What it does

It hooks into the agent/request-error waterfall:

• when the failure text matches an image-count limit (at most N images / maximum of N images / no more than N images / up to N images / too many images, and the word "image" must appear);
• it counts the total number of image occurrences on the current surface that are not yet omitted, total;
• it records an image/offload decision, picking the oldest max(1, total - (limit - safetyMargin)) occurrences in model request order;
• it returns { kind: 'retry' }, and the step is retried (without spending the provider retry budget).

It reuses the same official image/offload projection, so placeholder text, replay, token accounting, and KV cache semantics all match official offloading. Every retry omits at least one occurrence, so recovery always terminates: either the request fits, or there is nothing left to omit and the original failure stays terminal (no infinite retries).

Relationship to the official image-offload

Both write the same kind of image/offload event, so they do not conflict. This plugin does not depend on the official handler's registration order, only on the image/offload projection being registered; if the projection is not registered (the official plugin is disabled), append throws, and the plugin catches that and hands the failure back downstream without side effects.

Installation

# run this from the cloned repository directory
pwsh -File .\install.ps1                 # install into the desktop profile (default)
pwsh -File .\install.ps1 -Profile web    # install into the given profile

The script edits three places in the profile by hand (dependencies / dsh.profile.bundles / pnpm-lock.yaml) and copies a duplicate of node_modules (without running pnpm install, to avoid incidentally upgrading other ^ dependencies); before editing, it backs everything up to .backup\\ (keeping only the 5 most recent).

All insertion points use structural anchors ("dependencies": {, "bundles": [, and the packages: / snapshots: section headers in the lock file), so they do not depend on whether other plugins are present, nor on any machine-specific path. The DSH home directory comes from $env:DSH_HOME, falling back to ~\.dsh.

You must restart DSH after installing.

Uninstall:

pwsh -File .\install.ps1 -Uninstall

Configuration (optional)

No configuration is needed by default. When you do need it, override by id in the profile's cordis.patch.yml:

- id: image-count-guard
config:
defaultImageLimit: 20      # assumed limit when the error text gives no number
safetyMargin: 2            # margin kept after offloading, so adding one more image does not fail again
maxOffloadPerRetry: 80     # maximum number of images omitted per retry

Verification

1. Static: at the end, install.ps1 prints four checks: deps / bundles / node_modules / lock.
2. Offline unit tests: node --test test/guard.test.mjs (11 cases covering matching, selection, the upper bound on convergence rounds, and the no-side-effect boundary).
3. Assembly check (optional): mount the plugin into a temporary profile and run --dump-config to confirm the entry is assembled. Measured output contains # == dsh-image-count-guard / - id: image-count-guard, exit code 0.
4. Live: after restarting, open the stuck session and send "continue". With the plugin active, that session records an image/offload event, and the next request succeeds.

Diagnostics tool

# single session: image count, omitted count, raw bytes, base64 estimate, average image size
node tools\session-image-report.mjs `
"$env:USERPROFILE\.dsh\sessions\\session-\session.v4.jsonl.zstd"

# scan recent sessions, sorted by image count
node tools\session-image-report.mjs "$env:USERPROFILE\.dsh\sessions" --limit 20

Measured on two kinds of session:

The second row is why this plugin exists: an image count over the limit has nothing to do with the byte budget, and only count-based handling stops it.

Known limits

• Offloading cannot be undone: an omitted image appears as placeholder text from then on (including a read-only path to the attachment), which is inherent to the official image/offload semantics. If you need to see the image again, have the model call read_image once more.
• The error text must be in English and contain both image and a quantity phrase; when a new gateway wording appears, add an entry to LIMIT_PATTERNS in src/index.js.
• After changing src/index.js you must re-run install.ps1 to sync the node_modules copy, otherwise DSH keeps loading the old code.