dagger/dagger · error

load %dth value from %T: reconstruct result: %w

Error message

load %dth value from %T: reconstruct result: %w

What it means

Result[T].NthValue (dagql/cache.go:2945) fails at the last rehydration step: wrapSharedResultWithResolver is called to rebuild a live result (rebinding the resolver) from the stored shared result. Any failure inside that reconstruction — decoding the stored result, missing type/resolver registration, or an internal dagql error — is surfaced wrapped as 'reconstruct result'.

Source

Thrown at dagql/cache.go:2945

		srv := CurrentDagqlServer(ctx)
		if srv == nil {
			return nil, fmt.Errorf("load %dth value from %T: missing dagql server in context", nth, self)
		}
		clientMetadata, err := engine.ClientMetadataFromContext(ctx)
		if err != nil {
			return nil, fmt.Errorf("load %dth value from %T: current client metadata: %w", nth, self, err)
		}
		if clientMetadata.SessionID == "" {
			return nil, fmt.Errorf("load %dth value from %T: empty session ID", nth, self)
		}
		cache, err := EngineCache(ctx)
		if err != nil {
			return nil, fmt.Errorf("load %dth value from %T: current dagql cache: %w", nth, self, err)
		}
		touchSharedResultLastUsed(childShared, time.Now().UnixNano())
		retResAny, err := wrapSharedResultWithResolver(ctx, childShared, true, srv)
		if err != nil {
			return nil, fmt.Errorf("load %dth value from %T: reconstruct result: %w", nth, self, err)
		}
		if err := cache.trackSessionResult(ctx, clientMetadata.SessionID, retResAny, true); err != nil {
			return nil, fmt.Errorf("load %dth value from %T: claim cache-backed result: %w", nth, self, err)
		}
		return retResAny, nil
	}

	srv := CurrentDagqlServer(ctx)
	if srv == nil {
		return nil, fmt.Errorf("load %dth value from %T: missing dagql server in context", nth, self)
	}
	if parentCall.Type == nil || parentCall.Type.Elem == nil {
		return nil, fmt.Errorf("cannot get %dth value from %T without element type", nth, self)
	}
	req := &CallRequest{
		ResultCall: parentCall.fork(),
	}
	req.Type = req.Type.Elem.clone()

View on GitHub (pinned to 82ba2681db)

Solutions

1. Retry the operation with a fresh session — if the entry was reaped mid-load, the new call recreates the result normally.
2. Restart/clear the engine cache if entries are persistently corrupt (delete the engine cache data directory), then rerun.
3. Ensure all DAG types/resolvers are registered on the server before any results can be reconstructed (complete object registration at server setup).
4. Align engine versions between components so cached entries match the current schema/type registrations; check dagql server setup for missing Register calls.

Example fix

// before
// types registered lazily/late; cached results may resolve before Register happens
server := dagql.New(ctx, dagql.CacheConfig{})

// after
server := dagql.New(ctx, dagql.CacheConfig{})
core.RegisterTypes(ctx, server) // register everything up-front

// ensure types are registered before resolving cached results
func serverReady(ctx context.Context, srv *dagql.Server) error {
	// registration happens at startup; here just verify ctx inputs
	if _, err := engine.ClientMetadataFromContext(ctx); err != nil {
		return err
	}
	if _, err := EngineCache(ctx); err != nil {
		return err
	}
	return nil
}

func isReconstructionError(err error) bool {
	return err != nil && strings.Contains(err.Error(), "reconstruct result")
}

val, err := listResult.NthValue(ctx, nth)
if err != nil {
	if isReconstructionError(err) {
		// likely a reaped/racing entry: retry with a fresh call/session
		return retryNthValueWithFreshSession(ctx, listResult, nth)
	}
	return nil, err
}

When it happens

Trigger: wrapSharedResultWithResolver errors while re-wrapping a cache-backed shared result during NthValue: corrupt/evicted cache entry read mid-decode, result type whose resolver registration is missing on this build, or an internal error in the server's wrap path.

Common situations: Cache entries touched concurrently with GC/reaping (session released mid-load); engine binary mix where old cached entries reference types not registered after an upgrade/downgrade; genuinely corrupt engine cache state on disk.

Related errors

• wait: normalize returned result: %w
• load %s: %w
• failed parsing [%s], expected address format: [%s]
• use target SDK language: %s: %w
• unmarshal introspection json: %w

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/640a4ac88182efb2. Report an issue: GitHub.