diff --git a/packages/server/src/server/agent/provider-registry.ts b/packages/server/src/server/agent/provider-registry.ts
index 6bbeb1fb6..941156aa6 100644
--- a/packages/server/src/server/agent/provider-registry.ts
+++ b/packages/server/src/server/agent/provider-registry.ts
@@ -36,6 +36,10 @@ import {
   type AgentProviderDefinition,
 } from "./provider-manifest.js";
 
+function isNonEmptyStringArray(value: string[]): value is [string, ...string[]] {
+  return value.length > 0;
+}
+
 export type { AgentProviderDefinition };
 
 export { AGENT_PROVIDER_DEFINITIONS, getAgentProviderDefinition };
@@ -461,9 +465,11 @@ function addDerivedProviders(
     }
 
     if (override.extends === "acp") {
-      if (!override.command) {
+      if (!override.command || !isNonEmptyStringArray(override.command)) {
         throw new Error(`ACP provider '${providerId}' requires a command`);
       }
+      // Capture command in const for closure - TypeScript can't track type refinement inside closures
+      const command = override.command;
 
       resolvedProviders.set(providerId, {
         definition: createDerivedDefinition(
@@ -484,7 +490,7 @@ function addDerivedProviders(
         createBaseClient: (logger) =>
           new GenericACPAgentClient({
             logger,
-            command: override.command!,
+            command,
             env: override.env,
           }),
       });
diff --git a/packages/server/src/server/agent/providers/diagnostic-utils.ts b/packages/server/src/server/agent/providers/diagnostic-utils.ts
index 248cbe9bf..81778f86b 100644
--- a/packages/server/src/server/agent/providers/diagnostic-utils.ts
+++ b/packages/server/src/server/agent/providers/diagnostic-utils.ts
@@ -43,8 +43,9 @@ function truncateForDiagnostic(value: string): string {
   return `${trimmed.slice(0, DIAGNOSTIC_OUTPUT_CAP)}…(truncated)`;
 }
 
-function readStringProperty(error: object, key: string): string | undefined {
-  const value = (error as Record<string, unknown>)[key];
+function readStringProperty(error: Error, key: string): string | undefined {
+  if (!(key in error)) return undefined;
+  const value = (error as Error & Record<string, unknown>)[key];
   if (typeof value === "string") {
     return value;
   }
@@ -54,8 +55,9 @@ function readStringProperty(error: object, key: string): string | undefined {
   return undefined;
 }
 
-function readUnknownProperty(error: object, key: string): unknown {
-  return (error as Record<string, unknown>)[key];
+function readUnknownProperty(error: Error, key: string): unknown {
+  if (!(key in error)) return undefined;
+  return (error as Error & Record<string, unknown>)[key];
 }
 
 function pushIfNonEmpty(sections: string[], label: string, value: string | undefined): void {
diff --git a/packages/server/src/server/agent/providers/generic-acp-agent.ts b/packages/server/src/server/agent/providers/generic-acp-agent.ts
index 14ef3de73..682b16f61 100644
--- a/packages/server/src/server/agent/providers/generic-acp-agent.ts
+++ b/packages/server/src/server/agent/providers/generic-acp-agent.ts
@@ -5,7 +5,7 @@ import { ACPAgentClient } from "./acp-agent.js";
 
 interface GenericACPAgentClientOptions {
   logger: Logger;
-  command: string[];
+  command: [string, ...string[]];
   env?: Record<string, string>;
 }
 
@@ -13,20 +13,16 @@ export class GenericACPAgentClient extends ACPAgentClient {
   private readonly command: [string, ...string[]];
 
   constructor(options: GenericACPAgentClientOptions) {
-    if (options.command.length === 0) {
-      throw new Error("Generic ACP provider requires a non-empty command");
-    }
-
     super({
       provider: "acp",
       logger: options.logger,
       runtimeSettings: {
         env: options.env,
       },
-      defaultCommand: options.command as [string, ...string[]],
+      defaultCommand: options.command,
     });
 
-    this.command = options.command as [string, ...string[]];
+    this.command = options.command;
   }
 
   protected override async resolveLaunchCommand(): Promise<{ command: string; args: string[] }> {
diff --git a/packages/server/src/server/agent/providers/mock-load-test-agent.ts b/packages/server/src/server/agent/providers/mock-load-test-agent.ts
index d1c708aac..a99c4ed0e 100644
--- a/packages/server/src/server/agent/providers/mock-load-test-agent.ts
+++ b/packages/server/src/server/agent/providers/mock-load-test-agent.ts
@@ -166,9 +166,13 @@ function parseLargeAgentStreamPayloadPrompt(
   if (!Number.isFinite(bytes) || bytes <= 0) {
     return null;
   }
+  const kindValue = match[2]?.toLowerCase();
+  if (kindValue !== "diff" && kindValue !== "file" && kindValue !== "image") {
+    return null;
+  }
   return {
     bytes: Math.min(bytes, 1_000_000),
-    kind: match[2]?.toLowerCase() as LargeAgentStreamPayloadRequest["kind"],
+    kind: kindValue,
   };
 }
 
diff --git a/packages/server/src/server/agent/providers/tool-call-detail-primitives.ts b/packages/server/src/server/agent/providers/tool-call-detail-primitives.ts
index ab4dce148..b2adcabb1 100644
--- a/packages/server/src/server/agent/providers/tool-call-detail-primitives.ts
+++ b/packages/server/src/server/agent/providers/tool-call-detail-primitives.ts
@@ -859,19 +859,14 @@ export function toolDetailBranchByName<
     output: z.infer<OutputSchema> | null,
   ) => ToolCallDetail | undefined,
 ) {
-  return z
-    .object({
-      name: z.literal(name),
-      input: inputSchema.nullable(),
-      output: outputSchema.nullable(),
-    })
-    .transform((value) => {
-      const parsed = value as unknown as {
-        input: z.infer<InputSchema> | null;
-        output: z.infer<OutputSchema> | null;
-      };
-      return mapper(parsed.input, parsed.output);
-    });
+  const schema = z.object({
+    name: z.literal(name),
+    input: inputSchema.nullable(),
+    output: outputSchema.nullable(),
+  });
+  return schema.transform((value: z.infer<typeof schema>) => {
+    return mapper(value.input, value.output);
+  });
 }
 
 export function toolDetailBranchByToolName<
@@ -887,19 +882,14 @@ export function toolDetailBranchByToolName<
     output: z.infer<OutputSchema> | null,
   ) => ToolCallDetail | undefined,
 ) {
-  return z
-    .object({
-      toolName: z.literal(toolName),
-      input: inputSchema.nullable(),
-      output: outputSchema.nullable(),
-    })
-    .transform((value) => {
-      const parsed = value as unknown as {
-        input: z.infer<InputSchema> | null;
-        output: z.infer<OutputSchema> | null;
-      };
-      return mapper(parsed.input, parsed.output);
-    });
+  const schema = z.object({
+    toolName: z.literal(toolName),
+    input: inputSchema.nullable(),
+    output: outputSchema.nullable(),
+  });
+  return schema.transform((value: z.infer<typeof schema>) => {
+    return mapper(value.input, value.output);
+  });
 }
 
 export function toolDetailBranchByNameWithCwd<
@@ -916,19 +906,13 @@ export function toolDetailBranchByNameWithCwd<
     cwd: string | null,
   ) => ToolCallDetail | undefined,
 ) {
-  return z
-    .object({
-      name: z.literal(name),
-      input: inputSchema.nullable(),
-      output: outputSchema.nullable(),
-      cwd: z.string().optional().nullable(),
-    })
-    .transform((value) => {
-      const parsed = value as unknown as {
-        input: z.infer<InputSchema> | null;
-        output: z.infer<OutputSchema> | null;
-        cwd?: string | null;
-      };
-      return mapper(parsed.input, parsed.output, parsed.cwd ?? null);
-    });
+  const schema = z.object({
+    name: z.literal(name),
+    input: inputSchema.nullable(),
+    output: outputSchema.nullable(),
+    cwd: z.string().optional().nullable(),
+  });
+  return schema.transform((value: z.infer<typeof schema>) => {
+    return mapper(value.input, value.output, value.cwd ?? null);
+  });
 }