Claude Agent SDK vs Claude CLI: System Prompts and Output Consistency

Executive Summary

When sending the same message (e.g., "What is the capital of Norway?") through the Claude Agent SDK versus the Claude CLI (Claude Code), the system prompts accompanying these messages are fundamentally different. The CLI uses a modular system prompt architecture (~269 base tokens with additional context conditionally loaded based on features), while the SDK uses a minimal prompt by default. There is no guarantee of identical output between the two, even with matching configurations, due to the absence of a seed parameter and inherent non-determinism in Claude's architecture.

1. System Prompt Comparison

Claude CLI (Claude Code)

The Claude CLI uses a modular system prompt architecture with a ~269-token base prompt, with additional context conditionally loaded:

Component	Description	Loading
Base System Prompt	Core instructions and behavior	Always (~269 tokens)
Tool Instructions	18+ builtin tools (Write, Read, Edit, Bash, TodoWrite, etc.)	Always
Coding Guidelines	Code style, formatting rules, security practices	Always
Safety Rules	Refusal rules, injection defense, harm prevention	Always
Response Style	Tone, verbosity, explanation depth, emoji usage	Always
Environment Context	Working directory, git status, platform info	Always
Project Context	CLAUDE.md content, settings, hooks configuration	Conditional
Subagent Prompts	Plan mode, Explore agent, Task agent	Conditional
Security Review	Extended security instructions (~2,610 tokens)	Conditional

Key Characteristics:

Modular architecture with 110+ system prompt strings loaded conditionally
Base prompt is modest (~269 tokens), total varies by features activated
Includes extensive security and injection defense layers
Automatically loads CLAUDE.md files in the working directory
Session-persistent context in interactive mode

Claude Agent SDK

The Agent SDK uses a minimal system prompt by default containing:

Component	Description	Token Impact
Essential Tool Instructions	Only tools explicitly provided	Minimal
Basic Safety	Minimal safety instructions	Minimal

Key Characteristics:

No coding guidelines or style preferences by default
No project context unless explicitly configured
No extensive tool descriptions
Requires explicit configuration to match CLI behavior

2. What Each Interface Sends

Example: "What is the capital of Norway?"

Via Claude CLI

System Prompt: [modular, ~269+ base tokens]
  Base system prompt (~269 tokens)
  Tool instructions (Write, Read, Edit, Bash, Grep, Glob, etc.)
  Git safety protocols
  Code reference guidelines
  Professional objectivity instructions
  Security and injection defense rules
  Environment context (OS, directory, date)
  CLAUDE.md content (if present) [conditional]
  MCP tool descriptions (if configured) [conditional]
  Plan/Explore mode prompts [conditional]
  Session/conversation context

User Message: "What is the capital of Norway?"

Via Claude Agent SDK (Default)

System Prompt: [minimal]
  Essential tool instructions (if any tools provided)
  Basic operational context

User Message: "What is the capital of Norway?"

Via Agent SDK (with `claude_code` preset)

typescript

const response = await query({
  prompt: "What is the capital of Norway?",
  options: {
    systemPrompt: {
      type: "preset",
      preset: "claude_code"
    }
  }
});

System Prompt: [modular, matches CLI]
  Full Claude Code system prompt
  Tool instructions
  Coding guidelines
  Safety rules

// NOTE: Still does NOT include CLAUDE.md unless settingSources is configured

3. Customization Methods

Claude CLI Customization

Method	Command	Effect
Append to prompt	`claude -p "..." --append-system-prompt "..."`	Adds instructions while preserving defaults
Replace prompt	`claude -p "..." --system-prompt "..."`	Completely replaces the system prompt
Project context	CLAUDE.md file	Automatically loaded, persistent
Output styles	`/output-style [name]`	Apply predefined response styles

Agent SDK Customization

Method	Configuration	Effect
Custom prompt	`systemPrompt: "..."`	Replaces default entirely (loses tools)
Preset with append	`systemPrompt: { type: "preset", preset: "claude_code", append: "..." }`	Preserves CLI functionality + custom instructions
CLAUDE.md loading	`settingSources: ["project"]`	Loads project-level instructions
Output styles	`settingSources: ["user"]` or `settingSources: ["project"]`	Loads saved output styles

Configuration Comparison Table

Feature	CLI Default	SDK Default	SDK with Preset
Tool instructions	Full	Minimal	Full
Coding guidelines	Yes	No	Yes
Safety rules	Yes	Basic	Yes
CLAUDE.md auto-load	Yes	No	No*
Project context	Automatic	No	No*

*Requires explicit settingSources: ["project"] configuration

4. Output Consistency Guarantees

Critical Finding: NO Determinism Guaranteed

WARNING

The Claude Messages API does not provide a seed parameter for reproducibility. This is a fundamental architectural limitation.

Factors Preventing Identical Output

Factor	Description	Controllable?
Different system prompts	CLI vs SDK have different defaults	Yes (with configuration)
Floating-point arithmetic	Parallel hardware quirks	No
MoE routing	Mixture-of-Experts architecture variations	No
Batching/scheduling	Cloud infrastructure differences	No
Numeric precision	Inference engine variations	No
Model snapshots	Version updates/changes	No

Temperature and Sampling

Even with temperature=0.0 (greedy decoding):

Full determinism is NOT guaranteed
Minor variations can still occur due to infrastructure factors
Known bug: Claude CLI produces non-deterministic output for identical inputs

5. Achieving Maximum Consistency

To get the closest possible identical outputs between SDK and CLI:

Agent SDK Configuration

typescript

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

// Option 1: Use claude_code preset
const response = await client.messages.create({
  model: "claude-sonnet-4-20250514",
  max_tokens: 1024,
  // Match CLI system prompt as closely as possible
  system: "Your exact system prompt matching CLI",
  messages: [
    { role: "user", content: "What is the capital of Norway?" }
  ],
  // Use greedy decoding for maximum consistency
  temperature: 0
});

// Option 2: With Agent SDK query function
import { query } from "@anthropic-ai/agent-sdk";

for await (const message of query({
  prompt: "What is the capital of Norway?",
  options: {
    systemPrompt: {
      type: "preset",
      preset: "claude_code"
    },
    temperature: 0,
    model: "claude-sonnet-4-20250514",
    // Load project context like CLI does
    settingSources: ["project"]
  }
})) {
  // Process response
}

CLI Configuration

bash

# Match the SDK configuration as closely as possible
claude -p "What is the capital of Norway?" \
  --model claude-sonnet-4-20250514 \
  --temperature 0

Still Not Guaranteed

Even with perfectly matching configurations:

Output may differ between runs
Output may differ between SDK and CLI
No seed parameter exists to force reproducibility

6. Practical Implications

When to Use Each Interface

Use Case	Recommended Interface	Reason
Interactive development	Claude CLI	Full tool suite, project context
Programmatic integration	Agent SDK	Fine-grained control, embedding
Consistent API responses	Agent SDK + custom prompt	More control over system prompt
Batch processing	Agent SDK	Better for automation pipelines
One-off tasks	Claude CLI	Faster setup, immediate context

Design Recommendations

Don't rely on bit-perfect reproducibility
- Build applications robust to minor output variations
- Use structured outputs and validation
For production pipelines requiring consistency:
- Cache results when possible
- Use structured outputs with JSON schema validation
- Combine with deterministic logic and validation
- Consider multiple generations with consensus

For matching CLI behavior in SDK:

typescript

systemPrompt: {
  type: "preset",
  preset: "claude_code",
  append: "Your additional instructions"
},
settingSources: ["project", "user"]

7. System Prompt Token Impact

Configuration	Architecture	Notes
SDK (minimal)	Minimal default	Only essential tool instructions
SDK (claude_code preset)	Modular (~269+ base)	Matches CLI, varies by features
CLI (default)	Modular (~269+ base)	Additional context loaded conditionally
CLI (with MCP tools)	Modular + MCP	MCP tool descriptions add significant tokens

INFO

Claude Code uses a modular architecture with 110+ system prompt strings. The base prompt is ~269 tokens, with individual components ranging from 18 to 2,610 tokens depending on features activated.

Implication: The SDK's minimal default gives you more context for your actual task, but at the cost of Claude Code's full capabilities.

8. Summary Table

Aspect	Claude CLI	Agent SDK (Default)	Agent SDK (Preset)
System prompt	Modular (~269+ base)	Minimal	Modular (matches CLI)
Tools included	18+ builtin	Only if provided	18+ builtin
CLAUDE.md auto-load	Yes	No	No (needs config)
Coding guidelines	Yes	No	Yes
Safety rules	Full	Basic	Full
Temperature control	Yes	Yes	Yes
Determinism guarantee	No	No	No
Identical outputs?	N/A	No (vs CLI)	Closer, but no

9. Conclusion

Q: What system prompts accompany the same message in SDK vs CLI?

The CLI uses a modular system prompt architecture with a ~269-token base prompt and 110+ conditionally-loaded components (tool instructions, coding guidelines, safety rules, project context). The SDK uses a minimal default with only essential tool instructions, though it can be configured to match CLI behavior using the claude_code preset.

Q: Is there a guarantee of identical output?

No. Even with matching system prompts, identical inputs, and temperature=0, there is no guarantee of identical outputs due to:

Absence of a seed parameter in Claude's API
Floating-point arithmetic variations
Infrastructure-level non-determinism
Model architecture (Mixture-of-Experts) routing variations

Recommendation

Design systems to be robust to output variations rather than relying on deterministic behavior. For consistency-critical applications, use structured outputs, caching, and validation layers.

Claude Agent SDK vs Claude CLI: System Prompts and Output Consistency ​

Executive Summary ​

1. System Prompt Comparison ​

Claude CLI (Claude Code) ​

Claude Agent SDK ​

2. What Each Interface Sends ​

Example: "What is the capital of Norway?" ​

Via Claude CLI ​

Via Claude Agent SDK (Default) ​

Via Agent SDK (with claude_code preset) ​

3. Customization Methods ​

Claude CLI Customization ​

Agent SDK Customization ​

Configuration Comparison Table ​

4. Output Consistency Guarantees ​

Critical Finding: NO Determinism Guaranteed ​

Factors Preventing Identical Output ​

Temperature and Sampling ​

5. Achieving Maximum Consistency ​

Agent SDK Configuration ​

CLI Configuration ​

6. Practical Implications ​

When to Use Each Interface ​

Design Recommendations ​

7. System Prompt Token Impact ​

8. Summary Table ​

9. Conclusion ​

Sources ​

Claude Agent SDK vs Claude CLI: System Prompts and Output Consistency

Executive Summary

1. System Prompt Comparison

Claude CLI (Claude Code)

Claude Agent SDK

2. What Each Interface Sends

Example: "What is the capital of Norway?"

Via Claude CLI

Via Claude Agent SDK (Default)

Via Agent SDK (with `claude_code` preset)

3. Customization Methods

Claude CLI Customization

Agent SDK Customization

Configuration Comparison Table

4. Output Consistency Guarantees

Critical Finding: NO Determinism Guaranteed

Factors Preventing Identical Output

Temperature and Sampling

5. Achieving Maximum Consistency

Agent SDK Configuration

CLI Configuration

6. Practical Implications

When to Use Each Interface

Design Recommendations

7. System Prompt Token Impact

8. Summary Table

9. Conclusion

Sources