diff --git a/packages/llm/src/provider/anthropic-messages.ts b/packages/llm/src/provider/anthropic-messages.ts
index ea117ff706..70a4019efb 100644
--- a/packages/llm/src/provider/anthropic-messages.ts
+++ b/packages/llm/src/provider/anthropic-messages.ts
@@ -1,9 +1,8 @@
 import { Effect, Schema, Stream } from "effect"
-import { HttpClientRequest, type HttpClientResponse } from "effect/unstable/http"
+import type { HttpClientResponse } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
 import {
-  InvalidRequestError,
   Usage,
   type CacheHint,
   type FinishReason,
@@ -205,7 +204,7 @@ const decodeChunk = (data: string) =>
 const encodeTarget = Schema.encodeSync(AnthropicTargetJson)
 const decodeTarget = Schema.decodeUnknownEffect(AnthropicMessagesDraft.pipe(Schema.decodeTo(AnthropicMessagesTarget)))
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const baseUrl = (request: LLMRequest) => (request.model.baseURL ?? "https://api.anthropic.com/v1").replace(/\/+$/, "")
 
@@ -348,14 +347,11 @@ const prepare = Effect.fn("AnthropicMessages.prepare")(function* (request: LLMRe
 
 const toHttp = (target: AnthropicMessagesTarget, request: LLMRequest) =>
   Effect.succeed(
-    HttpClientRequest.post(`${baseUrl(request)}/messages`).pipe(
-      HttpClientRequest.setHeaders({
-        "anthropic-version": "2023-06-01",
-        ...request.model.headers,
-        "content-type": "application/json",
-      }),
-      HttpClientRequest.bodyText(encodeTarget(target), "application/json"),
-    ),
+    ProviderShared.jsonPost({
+      url: `${baseUrl(request)}/messages`,
+      body: encodeTarget(target),
+      headers: { "anthropic-version": "2023-06-01", ...request.model.headers },
+    }),
   )
 
 const mapFinishReason = (reason: string | null | undefined): FinishReason => {
@@ -529,7 +525,7 @@ export const adapter = Adapter.define<AnthropicMessagesDraft, AnthropicMessagesT
   protocol: "anthropic-messages",
   redact: (target) => target,
   prepare,
-  validate: (draft) => decodeTarget(draft).pipe(Effect.mapError((error) => invalid(error.message))),
+  validate: ProviderShared.validateWith(decodeTarget),
   toHttp: (target, context) => toHttp(target, context.request),
   parse: events,
 })
diff --git a/packages/llm/src/provider/bedrock-converse.ts b/packages/llm/src/provider/bedrock-converse.ts
index d4391e404f..280a21860b 100644
--- a/packages/llm/src/provider/bedrock-converse.ts
+++ b/packages/llm/src/provider/bedrock-converse.ts
@@ -2,11 +2,10 @@ import { EventStreamCodec } from "@smithy/eventstream-codec"
 import { fromUtf8, toUtf8 } from "@smithy/util-utf8"
 import { AwsV4Signer } from "aws4fetch"
 import { Effect, Option, Schema, Stream } from "effect"
-import { HttpClientRequest, HttpClientResponse } from "effect/unstable/http"
+import { HttpClientResponse } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
 import {
-  InvalidRequestError,
   Usage,
   type FinishReason,
   type LLMEvent,
@@ -225,7 +224,7 @@ const decodeChunk = (data: unknown) =>
 const encodeTarget = Schema.encodeSync(Schema.fromJsonString(BedrockConverseTarget))
 const decodeTarget = Schema.decodeUnknownEffect(BedrockConverseDraft.pipe(Schema.decodeTo(BedrockConverseTarget)))
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const region = (request: LLMRequest) => {
   const fromNative = request.model.native?.aws_region
@@ -401,9 +400,7 @@ const signRequest = (input: {
       return Object.fromEntries(signed.headers.entries())
     },
     catch: (error) =>
-      new InvalidRequestError({
-        message: `Bedrock Converse SigV4 signing failed: ${error instanceof Error ? error.message : String(error)}`,
-      }),
+      invalid(`Bedrock Converse SigV4 signing failed: ${error instanceof Error ? error.message : String(error)}`),
   })
 
 const toHttp = Effect.fn("BedrockConverse.toHttp")(function* (target: BedrockConverseTarget, request: LLMRequest) {
@@ -415,10 +412,7 @@ const toHttp = Effect.fn("BedrockConverse.toHttp")(function* (target: BedrockCon
   }
 
   if (isBearerAuth(request.model.headers)) {
-    return HttpClientRequest.post(url).pipe(
-      HttpClientRequest.setHeaders(baseHeaders),
-      HttpClientRequest.bodyText(body, "application/json"),
-    )
+    return ProviderShared.jsonPost({ url, body, headers: request.model.headers })
   }
 
   const credentials = credentialsFromInput(request)
@@ -427,11 +421,10 @@ const toHttp = Effect.fn("BedrockConverse.toHttp")(function* (target: BedrockCon
       "Bedrock Converse requires either a Bearer API key in headers or AWS credentials in model.native.aws_credentials",
     )
   }
+  // SigV4 signs the request including content-type; keep `baseHeaders` so the
+  // signed payload matches what `jsonPost` ultimately sends.
   const signed = yield* signRequest({ url, body, headers: baseHeaders, credentials })
-  return HttpClientRequest.post(url).pipe(
-    HttpClientRequest.setHeaders({ ...baseHeaders, ...signed }),
-    HttpClientRequest.bodyText(body, "application/json"),
-  )
+  return ProviderShared.jsonPost({ url, body, headers: { ...baseHeaders, ...signed } })
 })
 
 const mapFinishReason = (reason: string): FinishReason => {
@@ -666,7 +659,7 @@ export const adapter = Adapter.define<BedrockConverseDraft, BedrockConverseTarge
   protocol: "bedrock-converse",
   redact: (target) => target,
   prepare,
-  validate: (draft) => decodeTarget(draft).pipe(Effect.mapError((error) => invalid(error.message))),
+  validate: ProviderShared.validateWith(decodeTarget),
   toHttp: (target, context) => toHttp(target, context.request),
   parse: parseStream,
 })
diff --git a/packages/llm/src/provider/gemini.ts b/packages/llm/src/provider/gemini.ts
index 642ea81417..4994fcb477 100644
--- a/packages/llm/src/provider/gemini.ts
+++ b/packages/llm/src/provider/gemini.ts
@@ -1,10 +1,9 @@
 import { Buffer } from "node:buffer"
 import { Effect, Schema, Stream } from "effect"
-import { HttpClientRequest, type HttpClientResponse } from "effect/unstable/http"
+import type { HttpClientResponse } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
 import {
-  InvalidRequestError,
   Usage,
   type FinishReason,
   type LLMEvent,
@@ -151,7 +150,7 @@ const decodeChunk = (data: string) =>
 const encodeTarget = Schema.encodeSync(GeminiTargetJson)
 const decodeTarget = Schema.decodeUnknownEffect(GeminiDraft.pipe(Schema.decodeTo(GeminiTarget)))
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const baseUrl = (request: LLMRequest) =>
   (request.model.baseURL ?? "https://generativelanguage.googleapis.com/v1beta").replace(/\/+$/, "")
@@ -315,13 +314,11 @@ const prepare = Effect.fn("Gemini.prepare")(function* (request: LLMRequest) {
 
 const toHttp = (target: GeminiTarget, request: LLMRequest) =>
   Effect.succeed(
-    HttpClientRequest.post(`${baseUrl(request)}/models/${request.model.id}:streamGenerateContent?alt=sse`).pipe(
-      HttpClientRequest.setHeaders({
-        ...request.model.headers,
-        "content-type": "application/json",
-      }),
-      HttpClientRequest.bodyText(encodeTarget(target), "application/json"),
-    ),
+    ProviderShared.jsonPost({
+      url: `${baseUrl(request)}/models/${request.model.id}:streamGenerateContent?alt=sse`,
+      body: encodeTarget(target),
+      headers: request.model.headers,
+    }),
   )
 
 const mapUsage = (usage: GeminiUsage | undefined) => {
@@ -412,7 +409,7 @@ export const adapter = Adapter.define<GeminiDraft, GeminiTarget>({
   protocol: "gemini",
   redact: (target) => target,
   prepare,
-  validate: (draft) => decodeTarget(draft).pipe(Effect.mapError((error) => invalid(error.message))),
+  validate: ProviderShared.validateWith(decodeTarget),
   toHttp: (target, context) => toHttp(target, context.request),
   parse: events,
 })
diff --git a/packages/llm/src/provider/openai-chat.ts b/packages/llm/src/provider/openai-chat.ts
index 779fa3f28e..30db07d03d 100644
--- a/packages/llm/src/provider/openai-chat.ts
+++ b/packages/llm/src/provider/openai-chat.ts
@@ -1,9 +1,8 @@
 import { Effect, Schema, Stream } from "effect"
-import { HttpClientRequest, type HttpClientResponse } from "effect/unstable/http"
+import type { HttpClientResponse } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
 import {
-  InvalidRequestError,
   Usage,
   type FinishReason,
   type ContentPart,
@@ -164,7 +163,7 @@ interface ParserState {
 
 const decodeTarget = Schema.decodeUnknownEffect(OpenAIChatDraft.pipe(Schema.decodeTo(OpenAIChatTarget)))
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const baseUrl = (request: LLMRequest) => (request.model.baseURL ?? "https://api.openai.com/v1").replace(/\/+$/, "")
 
@@ -263,13 +262,11 @@ const prepare = Effect.fn("OpenAIChat.prepare")(function* (request: LLMRequest)
 
 const toHttp = (target: OpenAIChatTarget, request: LLMRequest) =>
   Effect.succeed(
-    HttpClientRequest.post(`${baseUrl(request)}/chat/completions`).pipe(
-      HttpClientRequest.setHeaders({
-        ...request.model.headers,
-        "content-type": "application/json",
-      }),
-      HttpClientRequest.bodyText(encodeTarget(target), "application/json"),
-    ),
+    ProviderShared.jsonPost({
+      url: `${baseUrl(request)}/chat/completions`,
+      body: encodeTarget(target),
+      headers: request.model.headers,
+    }),
   )
 
 const mapFinishReason = (reason: string | null | undefined): FinishReason => {
@@ -371,7 +368,7 @@ export const adapter = Adapter.define<OpenAIChatDraft, OpenAIChatTarget>({
   protocol: "openai-chat",
   redact: (target) => target,
   prepare,
-  validate: (draft) => decodeTarget(draft).pipe(Effect.mapError((error) => invalid(error.message))),
+  validate: ProviderShared.validateWith(decodeTarget),
   toHttp: (target, context) => toHttp(target, context.request),
   parse: events,
 })
diff --git a/packages/llm/src/provider/openai-compatible-chat.ts b/packages/llm/src/provider/openai-compatible-chat.ts
index 268f31d1e8..0b1836393a 100644
--- a/packages/llm/src/provider/openai-compatible-chat.ts
+++ b/packages/llm/src/provider/openai-compatible-chat.ts
@@ -1,8 +1,7 @@
 import { Effect, Stream } from "effect"
-import { HttpClientRequest } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
-import { InvalidRequestError, ProviderChunkError, type LLMError, type LLMRequest } from "../schema"
+import { ProviderChunkError, type LLMError, type LLMRequest } from "../schema"
 import { OpenAIChat, type OpenAIChatTarget } from "./openai-chat"
 import { families, type ProviderFamily } from "./openai-compatible-family"
 import { ProviderShared } from "./shared"
@@ -20,7 +19,7 @@ export type ProviderFamilyModelInput = Omit<OpenAICompatibleChatModelInput, "pro
   readonly baseURL?: string
 }
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const isStringRecord = (value: unknown): value is Record<string, string> =>
   typeof value === "object" && value !== null && !Array.isArray(value) && Object.values(value).every((item) => typeof item === "string")
@@ -42,14 +41,11 @@ const toHttp = (target: OpenAIChatTarget, request: LLMRequest) =>
   Effect.gen(function* () {
     const url = completionUrl(request)
     if (!url) return yield* invalid("OpenAI-compatible Chat requires a baseURL")
-
-    return HttpClientRequest.post(url).pipe(
-      HttpClientRequest.setHeaders({
-        ...request.model.headers,
-        "content-type": "application/json",
-      }),
-      HttpClientRequest.bodyText(ProviderShared.encodeJson(target), "application/json"),
-    )
+    return ProviderShared.jsonPost({
+      url,
+      body: ProviderShared.encodeJson(target),
+      headers: request.model.headers,
+    })
   })
 
 const mapParseError = (error: LLMError) => {
diff --git a/packages/llm/src/provider/openai-responses.ts b/packages/llm/src/provider/openai-responses.ts
index 5e38e7cf3e..1ee02c703e 100644
--- a/packages/llm/src/provider/openai-responses.ts
+++ b/packages/llm/src/provider/openai-responses.ts
@@ -1,9 +1,8 @@
 import { Effect, Schema, Stream } from "effect"
-import { HttpClientRequest, type HttpClientResponse } from "effect/unstable/http"
+import type { HttpClientResponse } from "effect/unstable/http"
 import { Adapter } from "../adapter"
 import { capabilities, model as llmModel, type ModelInput } from "../llm"
 import {
-  InvalidRequestError,
   Usage,
   type FinishReason,
   type LLMEvent,
@@ -149,7 +148,7 @@ interface ParserState {
   readonly tools: Record<string, ToolAccumulator>
 }
 
-const invalid = (message: string) => new InvalidRequestError({ message })
+const invalid = ProviderShared.invalidRequest
 
 const baseUrl = (request: LLMRequest) => (request.model.baseURL ?? "https://api.openai.com/v1").replace(/\/+$/, "")
 
@@ -239,13 +238,11 @@ const prepare = Effect.fn("OpenAIResponses.prepare")(function* (request: LLMRequ
 
 const toHttp = (target: OpenAIResponsesTarget, request: LLMRequest) =>
   Effect.succeed(
-    HttpClientRequest.post(`${baseUrl(request)}/responses`).pipe(
-      HttpClientRequest.setHeaders({
-        ...request.model.headers,
-        "content-type": "application/json",
-      }),
-      HttpClientRequest.bodyText(encodeTarget(target), "application/json"),
-    ),
+    ProviderShared.jsonPost({
+      url: `${baseUrl(request)}/responses`,
+      body: encodeTarget(target),
+      headers: request.model.headers,
+    }),
   )
 
 const mapUsage = (usage: OpenAIResponsesUsage | undefined) => {
@@ -396,7 +393,7 @@ export const adapter = Adapter.define<OpenAIResponsesDraft, OpenAIResponsesTarge
   protocol: "openai-responses",
   redact: (target) => target,
   prepare,
-  validate: (draft) => decodeTarget(draft).pipe(Effect.mapError((error) => invalid(error.message))),
+  validate: ProviderShared.validateWith(decodeTarget),
   toHttp: (target, context) => toHttp(target, context.request),
   parse: events,
 })
diff --git a/packages/llm/src/provider/shared.ts b/packages/llm/src/provider/shared.ts
index f9e347d04e..ca4996ede6 100644
--- a/packages/llm/src/provider/shared.ts
+++ b/packages/llm/src/provider/shared.ts
@@ -1,7 +1,7 @@
 import { Cause, Effect, Schema, Stream } from "effect"
 import * as Sse from "effect/unstable/encoding/Sse"
-import type { HttpClientResponse } from "effect/unstable/http"
-import { ProviderChunkError } from "../schema"
+import { HttpClientRequest, type HttpClientResponse } from "effect/unstable/http"
+import { InvalidRequestError, ProviderChunkError } from "../schema"
 
 export const Json = Schema.fromJsonString(Schema.Unknown)
 export const decodeJson = Schema.decodeUnknownSync(Json)
@@ -114,4 +114,41 @@ export const sse = <Chunk, State, Event>(input: {
   readonly onHalt?: (state: State) => ReadonlyArray<Event>
 }): Stream.Stream<Event, ProviderChunkError> => framed({ ...input, framing: sseFraming })
 
+/**
+ * Canonical `InvalidRequestError` constructor. Lift one-line `const invalid =
+ * (message) => new InvalidRequestError({ message })` aliases out of every
+ * adapter so the error constructor lives in one place. If we ever extend
+ * `InvalidRequestError` with adapter context or trace metadata, the change
+ * lands here.
+ */
+export const invalidRequest = (message: string) => new InvalidRequestError({ message })
+
+/**
+ * Build a `validate` step from a Schema decoder. Replaces the per-adapter
+ * lambda body `(draft) => decode(draft).pipe(Effect.mapError((e) =>
+ * invalid(e.message)))`. Any decode error is translated into
+ * `InvalidRequestError` carrying the original parse-error message.
+ */
+export const validateWith =
+  <A, I, E extends { readonly message: string }>(decode: (input: I) => Effect.Effect<A, E>) =>
+  (draft: I) =>
+    decode(draft).pipe(Effect.mapError((error) => invalidRequest(error.message)))
+
+/**
+ * Build an HTTP POST with a JSON body. Sets `content-type: application/json`
+ * automatically (callers can't override it — every adapter today places it
+ * last so caller headers win on everything else) and merges caller-supplied
+ * headers. The body is passed pre-encoded so adapters can choose between
+ * `Schema.encodeSync(target)` and `ProviderShared.encodeJson(target)`.
+ */
+export const jsonPost = (input: {
+  readonly url: string
+  readonly body: string
+  readonly headers?: Record<string, string>
+}) =>
+  HttpClientRequest.post(input.url).pipe(
+    HttpClientRequest.setHeaders({ ...input.headers, "content-type": "application/json" }),
+    HttpClientRequest.bodyText(input.body, "application/json"),
+  )
+
 export * as ProviderShared from "./shared"